I don't remember much about the manga but it's about an assassin who never manage to kill their target because they all die in various accidents beforehand.
@Eliestal don't think this is it but it is the closest I can think of. The guy steals a persons luck and then they die by some accident. If it is I leave it to you to @ just...
https://mangadex.org/title/2333/luck-stealer
for those interested, came across this one while looking, assassin comedy. Only 9 chapters so far, but it's a quick read and hilarious.
https://mangadex.org/title/31843/kanako-s-life-as-an-assassin
@Eliestal actually I'm kind of disappointed. The idea of him not managing to kill them and them dying accidentally rather them him being the cause, through the use of his power, could be a funny short series. Maybe 20 or 30 chapters. He goes to kill some one they accidentally die and the mob thinks he's brilliant. It keeps happening. You could have the police eventually find out about him, investigate, find proof of the hit payments and arrest him and have him go to jail for the deaths of the people he was paid to kill but actually didn't. Could be a funny short series.