A practical example
Think about Lord Elrond in The Lord of the Rings. Pretend like you don't know the story, and I tell you that there is a character who:
- Holds grudges over singular mistakes that were made generations ago and actively dismisses the future generations of their ally's plea for help because of it.
- Overgeneralizes other races. If one member of the other race makes a mistake, all others are obviously going to do the same.
- Prohibits his child from dating someone from another race and living in the conditions of this other race.
You wouldn't particularly peg this guy as the good guy, right?
What makes Elrond not evil is the fact that he's effectively immortal, which means that Elrond has a different view on time-sensitive issues.
- Elrond was there when the mistake was made generations ago. To Elrond, that's something that happened in his life. He forgets that humans have had several generations since then and that the descendants of Isildur cannot be blamed for Isildur's mistake.
- Elrond did not immediately spring to action when called upon, because he has no sense of urgency. As an immortal being, he wouldn't be particularly bothered about taking 50 years to do something, whereas humans will effectively live their entire lives in that timespan.
- Elrond cannot process the idea of fleeting mortality when it affects him. 50 years is the blink of an eye to him. And his daughter has chosen to give up her immortality and will live the rest of her life in the blink of an eye. To a being with a nigh-infinite lifespan, this is the equivalent of Arwen telling Elrond she's going to kill herself in the "near" future. Effectively, Elrond is a parent who will see their child die and thus lashes out emotionally at that prospect (blaming Aragorn for what is Arwen's choice).
The point is not to deeply analyze the character of Elrond, but I wanted to show you how a character who is numb to old age behaves differently (or with different interest) than a character who does age.
The general approach
How can I explain the radical change in personality when a person has been transformed into a vampire?
In general, a person responds to things that positively/negatively impact them. Often, people do something they don't innately want to do because the benefits outweigh the drawbacks.
- A religious person may choose to follow their religion's commandments (even the ones they disagree with) because they want to avoid eternal damnation.
- An old man who very quickly breaks his bones will learn to avoid physical activities. Even if he really likes physical activities, he wants to avoid the pain.
- A young person can refuse to smoke or partake in unhealthy behavior because they want to elongate their lifespan.
But when you take away the punishment, there's no need to still do the thing:
- When the religious person finds out there is no eternal damnation (heaven for everyone, or no afterlife for anyone), they may stop complying with commandments they don't agree with.
- When the old man get an unbreakable skeleton, he may pick up physical activities again.
- When the behavior does not shorten their lifespan, the young person no longer has a reason to avoid the behavior.
This can be gradual or immediate, based on the character. A good guy will hold on to their morality, but they may lose sight of it over an extended period of time. A bad guy will almost revel in the opportunity to do the thing they can now freely do.
However, you should choose the right response for the right character. Each of your given examples carries a subcontext of the person making this argument:
- Superiority is a fact we cannot deny and should not avoid.
- This is how nature works.
- The long-term goal outweight the short-term goal.
- No one is more important than me.
They are all valid justifications for vampiric behavior, but you can't just give any character any justification (unless you're specifically going for a "you never know what a person is truly like" vibe with your story).
Just to oversimplify, you could stereotypically assign these justifications to:
- A white supremacist
- A biologist or survival expert
- A cold and logical person (we come back to Elrond here)
- A vain politician or celebrity
This is despite them being humans once and still having memory of human life.
I always think back to Sophie. She was a classmate of mine. In class, there were a few advanced students who did not need test revisions (scoring over 90%), but a lot of others did need guidance (scoring under 60%). Because of this, the teacher made a rule that during test revision, those scoring >85% could instead play a board game.
Usually, this was the same 5 students. But Sophie was an infrequent addition, she always scored around the 80%-90% mark.
- When Sophie scored <85%, she complained that it's unfair for some students to not have to do the same work as her/the rest. She was the one to loudly complain.
- When Sophie scored >85% and someone else made that same argument, Sophie responded that she clearly didn't need revision and shouldn't be made to do it. Again, Sophie was the only one to take center stage to defend her opinion, the other "passing" students were relatively indifferent to it compared to her.
People's opinion on categories changes when people change categories. This is normal human behavior, even if an outside observer can plainly see the hypocrisy.
Someone who is actively protesting tax cuts for the rich (because they are poor) might, when becoming rich themselves, start arguing that the rich shouldn't need to pay more taxes than other people.
Or, a human who advocates enthnically cleansing the world of vampires, may let go of that notion when they themselves become a vampire and possibly start arguing the opposite.
This is effectively what Magneto does to Senator Kelly (an anti-mutant political leader) in X-Men: he makes him a mutant, and Kelly changes his rhetoric overnight because he now fears becoming subjected to the system he tried to put in place.