9. Kratos - God Of War
With God of War's Kratos, we're clearly using the term "hero" pretty loosely here, because in his original iteration he's really a tragically doomed anti-hero at best, if not a straight-up mass murderer.
In the original game, Kratos made a deal with the God of War, Ares, to serve under him in exchange for sparing his life during battle and granting him god-like powers.
Kratos was then tricked into killing his own wife and daughter, and it's also made clear that he killed a lot of people while serving Ares prior to this. He even tellingly says, "I killed many who were deserving, and many who were not."
From here, Kratos ends up slaughtering over two-dozen gods - including Ares - throughout the series.
In many cases this caused an untold amount of indirect death, such as when he killed Poseidon and flooded the Mediterranean, murdered Helios and plunged Greece into darkness, and unleashed a plague by slaughtering Hermes.
Had Kratos stopped after killing Ares, perhaps we'd be more willing to go easy on him as the pawn of a god, but the guy just didn't know when to quit.