A term that would define someone who'd prefer to inflict the most pain on someone they sought revenge on rather killing quickly [closed]
You could describe them as "cruel" or "inhumane." If you want to emphasize the revenge aspect, you could call them "vengeful." To incorporate the idea that they aren't satisfied with an eye for an eye, you could call them "implacably vengeful."
The term sadist or sadism or sadistic works for me.
The deriving of pleasure from cruelty; thus, a person deriving pleasure from cruelty is a sadist. Adjectival form is sadistic.
I guess you could say that sadism is an extreme form of schadenfreude, which is the delight one feels in the misfortune or pain of someone else.