steve’s grief manifesting as an anger that he even brings with him to the modern world. this is how he coped; by channeling that pain into something greater and using it as a weapon. remember that dark side comment by tony? this is a peek at it. when bucky is involved steve turns himself into a weapon for bucky. he never quite let himself properly grieve and even now that bucky is back he still holds onto that anger because someone is once again trying to take someone who is his.
THIS
Steve’s incandescent rage (especially where Bucky is concerned) will always be the most beautiful and terrifying in thing in the world.