Source: lowbrowcomics.com

4. Crossbones

Brock Rumlow, alias Crossbones, is stone cold scary.

A gang leader in his youth, Rummy eventually entered Taskmaster's crime school, becoming a trained mercenary and murder specialist. He bounced around a bit under various names, working for various villains. With Taskmaster, he was Bingo Brock. With the imposter Red Skull, he was Frag, but it was with the real Red Skull that he took on a new codename: Crossbones.

He worked as a hired gun for the Skull for a lengthy stretch of time, pausing only to make an attempt on Gambit's life at New Son's behest. But he was drawn back into to Red Skullduggery (See what I did there? I'm – no joke – proud of that joke.) for a role in the death of Steve Rogers.

He fired at Steve with a sniper rifle and shot him in the shoulder. He wasn't the one to kill Steve, but he had a big hand in it. Impressive for someone who used to call himself Bingo Brock.