just because she's defeated doesn't mean she's killed, dwight as a sage doesn't go on a killing rampage, you probably could made a tame shounen manga out of dwight and claire (the hero) journey to defeat demon lord, i deduce that mainly from seeing pieces of flashback lich dwight have
which chapter that makes you think the succubus are the hero really? the succubus introduction are pretty much her trying to get to safety with the remnants of the former demon lord army, so either you skim it or muddled with some other manga you're reading or both