In the context of the high-action scenes preceding it, this feels too slow. Notice how Vader has already entered the space and is examining the bodies even as his theme continues to play, whereas the original has just a few dissonant notes which hold as he investigates the scene and the same musical idea picks up again as he leaves. It just fits the scene much better.

That’s not to say the Imperial March wouldn’t work here at all, just that it would need to be a shorter, sharper sting. It would also need to be propagated throughout the film with increasing depth and modulation to really act as Vader’s theme.