In my mind, the scariest and probably the most powerful hero is probably Ant-Man.
One of the reasons would be that he can shrink to microscopic size, enter through your skin, and then grow again inside his victims exploding them as if it's a Mortal Kombat finisher move.
Another would be that he can travel through his victim's heart and punch it. One of the most terrifying facts about this is that while his size is that small, the force that his punches give is still as if he were at normal size (due to the nature of Pym Particles as explained in the movie).
However, was there ever a comic where the authors have explored this ability of Ant-Man? When would this not work? Is there a reason why Hank Pym or other iterations of Ant-Man avoid doing this?