And don't forget that after seeing his father burn alive he stood there watching the flames, blowing at the embers on the wind and enjoying the spectacle. Hardly a normal reaction.
I prefer Imaginality to reality.

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The whole film is told from Seth's POV, so it's an untrustworthy narrator, the teens in the black car don't exist, its all Seth.

I'd say he was both psychopathic and psychotic (for his delusions).
I perused some earlier posts and surprised that no one brought these points up:
Remember what Seth did to the frog at the beginning? What did his mother do to him with the cups of water? Seth's father had a vampire obsession, and Seth did as well. One sorta has to wonder whether his father was also a serial killer before the self-immolation.
Finally, there's the issue of dissociation, which is common with psychiatric disorders. The mentally ill person perceives everyone else as crazy/eccentric, while he she engages in either ghastly or abhorrent behavior as if it's an everyday norm.

Have you even considered that he may have disliked/hated her because she was killing his brother assumingly because she was a "vampire"?
I don't believe she was a real vampire anyway.
She walked in the sun, and those 'serialkillers' could never have killer her if she really was one. I didn't see a wooden steak or her head chopped off.
"
Gar nicht so bel, du kleine Schlampe. Man sieht sich immer zweimal, Kleine.
"