One of the earliest and most gruesome "hero-killing-villain" deaths was performed by Captain America (Steve Rogers) in a battle against the vampire, Baron Blood.

The Captain performs the gruesome deed, complete with decapitation in Captain America #254 (1981).

As my runner-up and dear to my heart (because I despised the Green Goblin) I present the Amazing Spiderman, Vol 1, #122 (1973) who was involved in and could be held culpable in the death of the Green Goblin.

SETUP
As I said, he could be held culpable though he did not deal the killing blow...just a whole bunch of them which lead up to the Goblin's demise.
The Green Goblin is responsible for the death of Peter Parker's first love Gwen Stacy after throwing her off a bridge. Spider-Man was unable to save her and takes his rage out on the Green Goblin looking for all intents and purposes a man possessed with a murderous rage.

The Goblin looks doomed. Spider-Man regains his sanity and stays his hand. The writers could not have Spider-Man going down that dark path, but someone would have to pay for Gwen Stacy's death. The fans were outraged. For the Goblin, this meant his time was up...
