Yes they are given the right circumstance.. if they had time to prepare As most people know Bruce Banner is the human form of Hulk (so to speak) and as a baseline human he's quite easy to kill (given that Banner didn't have some kind of technological protection like a forcefield against Hulk's enemies who know about his dual state). One surprise attack and he's dead. Steve Rogers aka Captain America is a peak condition human and very resilient but if you manage to hit him while his Shield doesn't protect him he can be killed (and i guess even the best bodyarmor won't protect against a full force blast from a Sentinel). Of course Captain America is a brilliant tactician and field leader so he knows about his weakness and would conduct the fight accordingly.. even retreat if the odds are seriously against him but it would be easy to write it in a way where he gets hit in the back while he threw his shield to take out another Sentinel. Every Superhero has weaknesses which can be exploited.. many of them are quite fragile because they are mostly human protected only by their powers, teamwork and experience but apply an intelligent, well prepared attack with overwhelming firepower and you can take them down. Comics don't do that of course on a regular basis because they'd soon run out of Heroes but this is also why Days of Future Past is so popular because the unthinkable happened on a grand scale.