You want to kill Captain America and Bucky, they're both tied up and unconscious, you have plenty of guns. Don't over-think this.

I think it was in Grant Morrisons "Club of Heroes" story that one of the villains laments the sheer unimaginativeness of villains these days, something along the lines of "These days, it's all a bullet in the head and off to the bar" and at which point he unveils a fairly impressively complicated deathtrap. Must see if I can find that issue.