A person who is told by 2 eidim, “don’t kill or we will put you to death” and kills anyway is the only case where beis din actually gives the death penalty. Would you consider such a person “sane”? Why couldn’t he at least wait until the next day when the eidim are gone? The Torah obviously considers such a person to be a rasha, not a shoteh. Everyone has a yetzer hara. Some people allow the yetzer hara to control them and they can do horrendous things. This makes them evil, not insane
That said, it still could be that Levi Aron is insane, but don’t use his crime as proof.