Read this quite a while ago, protagonist gets summoned by some shady magicians, but with the power he got while summoning he kills them all.
Probably the post above but these two do have something like that as well: Isekai Shihai no Skill Taker: Zero kara Hajimeru Dorei Harem Wortenia Senki
you should post in the manga section: it's not summoned slaughter because that doesn't have a manga in English Kalto's 2 suggestions might be correct, but it might also be: Nidome no Yuusha I think there are other possibilities too, so can you remember anything else?
maybe Kuro no Maou. He got summond, experimented/torured on so he aqired magic. Many people die in this, and not only the bad people