Can a werewolf kill himself?
Werewolf is the name of a family of games, not a specific game. So the answer will vary depending on the rules of the specific game. I think the more common answer would be "No, because they have to pick a 'villager', and the werewolves aren't 'villagers'."

e.g.,:

https://www.playwerewolf.co/rules

But other rulesets vary in their phrasing, with some even explicitly allowing the selection of a werewolf.