The difference being that the scene from Se7en actually went somewhere. Mills executed John Doe.
Tony just ripped off Bucky's prosthetic arm. Good job, Marvel.
Tony should have killed Bucky with Steve watching, unable to help. Then have the moment Steve beats Tony down but refuses to kill him.
That would have been impacting. That would have left the theater in silence. It would crush the bond and respect between Steve and Tony forever. Irreparable damage. I would have taken that over Cap's death.
But no, Marvel/Disney can't even keep Coulson dead.
**** it, at least over at FOX we have Magneto brilliantly executing Shaw.