Red Skull, a couple of times, but let's face it, when doesn't the Red Skull die? Same thing with Machinesmith, who I guess technically just got killed off, but if he came back, would anyone even need an explanation?
I forget, did Man Without a Face die? But even if he was technically a Golden Age character, for all intents and purposes, he was new.
The other guys D-Man killed, but I'm pretty sure they were all replacements of replacements of replacements.
Mother Night. Bucky definitely killed of Mother night.
Lukin, but Bru created him, so whatever.
All I can think of. So of ones that really count, maybe D-Man (although I'm waiting till the conclusion), Mother Night, Nomad.