He's Not the Same

[Note: After watching considerably more of this show, I've found out that I really misunderstood the vampires. Angel was not Spike's sire, Spike, as far as I can tell, didn't actually know Angel before he got his soul back, and Spike didn't really like Angel when he was soulless.]
I've been watching Buffy the Vampire Slayer a lot lately. And I've noticed something interesting about that show.
In that series, when a person becomes a vampire, in addition to changing quite a lot physically, xe has a significant personality change. This is explained as losing xyr soul. One vampire, Angel, had the reverse happen to him as well, because a spell done on him restored his soul to his body. At that point, he had a major personality change as well.
How do loved ones react to that kind of change? There are two examples I've seen so far in the series:
Xander and Jesse:
In the very first episode, Buffy befriends two boys named Xander and Jesse. These two are described as having been good friends for a long time. During a vampire attack, Jesse gets kidnapped, and by the time Buffy, Xander and another friend come to rescue him, he's been turned into a vampire. He's not overly mean to Xander after that, but makes it quite clear that he's siding with the vampires against Xander and Buffy.
Buffy, who wasn't very close to Jesse, immediately turns against him at that point, telling Xander to 'look at him not as your friend, but as the creature who killed him'. Although Xander finds this very difficult, he accepts her advice and ends up killing Jesse*. (It is possible, however, that Xander wasn't actually very close to Jesse, because he is never mentioned again in later episodes.)
Spike and Angel:
Angel, before he regained his soul, turned Spike into a vampire. As Spike's 'sire', therefore, he is very important to Spike, basically the vampire version of a parent. Then a spell restores Angel's soul, and he turns against Spike, even helping Buffy when she tries to kill Spike.
Spike, unlike Xander, does not turn against Angel. Although he takes measures necessary to ensure his own safety when Angel is a danger to him (by hitting Angel and pushing Angel away from him), Spike makes it quite clear that, even quite awhile after Angel was affected by that spell, Spike still cares about him.
The idea that someone has fundamentally changed as a person, to the point that they're no longer the person they used to be, is not only seen in fantasy. Many real people are viewed this way, such as developmentally disabled people who experience regressions, people with certain 'mental illnesses' such as schizophrenia, and people with brain injuries. And loved ones often treat the person who's changed fairly badly, in ways that they'd never have treated that person before their change (in another Buffy episode, Xander gets possessed by a hyena and they end up locking him in a cage and refusing to let him out). From the perspective of the person's loved ones, this is 'for their own good', and the unpleasant things they do to the person who's changed are really being done to the illness or whatever that's changed them (just like how Buffy rationalizes killing Jesse in her discussions with Xander). But from the perspective of the person who's changed:

"'I want the real you back' prompts the questions, 'Do you know me? Would you love me if you found out this is the real me? Aren’t you supposed to love who I am, not who you imagine?'
...
By the way, this is even true of changes that are traditionally viewed as very negative. I have known of many people who, after brain damage, have all their friends say they want them to be the person they were before the brain damage. It doesn’t happen. It’s not real. It hurts them. It’s not that they wanted to get knocked on the head, it’s that who they are now happens to be a person who got knocked on the head, not the imaginary person that didn’t."

Ironically, in this particular way, the vampires in the Buffy series (except for Angel) come off as morally better than the humans. Spike and the other vampires who knew and cared about Angel before he got his soul mostly treat him fairly well. Darla, who I suspect was Angel's sire (I read a book about Angel before, but don't remember much of it), was trying to coax him to drink blood with her. Spike hits Angel when Angel tries to trick him in order to help Buffy, but makes it quite clear that if Angel wants to have a real relationship with Spike, he'll welcome that. They don't seem to be doing things to Angel that they'd have been unwilling to do to him when he didn't have his soul - unlike the humans in the story. Because the vampires in that universe are bad in many ways, but they are certainly capable of being loyal to a loved one.

* Not entirely intentionally, though. He was threatening Jesse with a stake, trying to kill Jesse but hesitating because it felt emotionally like killing his own friend, and then someone bumped Jesse from behind and pushed him onto the stake. However, it's likely that if he'd had more time to decide, he would have killed Jesse eventually.

Growing Backwards?

In the book When Autism Strikes: Families Cope with Childhood Disintegrative Disorder, one parent discusses an analogy that 'your first child runs through you like a river'. She says that we all know a river can go astray, dry up, or get polluted, but rivers don't go backward. Yet that's what her son Jordan seemed to do when his regression started.
But children with CDD and other forms of regressive autism don't really grow backwards. If someone was growing backwards, then they'd act like they did at a younger age. When you have a hyperverbal, sociable 2 year old growing into a shy, somewhat quirky 4 year old who then becomes an aloof, obsessive, echolalic 6 year old, that 6 year old is clearly not like he was at any age before 4. Before 2 years, he was a sociable nonverbal child who couldn't walk becoming a sociable walking child who used single words becoming a sociable running child who used communicative sentences. He was never echolalic, obsessive or aloof.
This is part of the mental age fallacy. An autistic child at any age is not the same as a same-age, younger or older child. And an autistic child who regressed is not like they were at a younger age.
So far, I've just been looking at outside appearances. If you look at what is going on inside the child, the analogy of growing backward fits even less.
Amanda Baggs described 'regression' like this:

"it’s your entire brain shifting around, focusing on some things that a lot of people find unimportant or bad, failing to focus on some things that a lot of people find important or good. Imagine that the things it is focusing on are exactly what you need to be focusing on. You are becoming the sort of person you need to be."

This sounds pretty much like a lot of the kind of changes that normally go with growing up. Not growing backward.
She also talks about how someone with brain damage can change afterward, which is another way you could view some cases of autistic regression. In that case, you're certainly not growing backward, and if your behaviour post-brain injury gets viewed at all as like a younger version of yourself, it's purely coincidence. Saying a person with brain damage is 'growing younger' because they have lost skills they previously had is like saying a 4 year old who went deaf and then forgot how to talk has 'grown younger' because xyr communication skills superficially resemble a younger child. The kid's expressive language skills may be said to be at a 1 year old level, but firstly, their receptive language skills are poorer than a 1 year old while their motor and cognitive skills are much better, and secondly, when that kid was 1, xe could hear. Xe couldn't speak for very different reasons than now.
Here's an analogy I thought of awhile back:
Three children are traveling along three paths, that appear to be pretty close to identical. They are traveling at the same rate.
One kid is normal - xyr path may be straight or winding, but other people view that as the expected path for a child to take.
The second child seems to be heading along that path, but then xyr path turns when the other one doesn't, or doesn't turn when the other one does, and you realize xe was actually heading along a different path all along. This is what autistic regression usually is like.
The third child really is heading down the usual path, but then xe gets jostled onto a different path and start walking along it instead. This is more like brain damage.
None of those children ever went backward. They never retraced a part of the path they'd seen before. Instead, they all kept going forward, but the path ahead diverges from the other two children in various ways.