Same goes for Kaido and BM, they were too big to be left alive but at the same time "Luffy doesn't kill". But the plot does so we get that ridiculous misfortune that they both fell in the magma pit below Wano AND the volcano erupted just when they were inside the magma chamber, what a bad luck (contrary to somebody who when dies actually comes back as a god). This of course doesn't exclude the chance that Paido and Pig Mom will reappear in a cover story in X time.
For Im the catch could simply be that he lived far longer than he should due to some unnatural method and when Im is defeated this method fails to function as well and he is eroded away (just like what happened to Saturn). So again Luffy doesn't kill his opponent but the plot does it for him (without even stretching it so much like for a Kaido and BM).