俺がもっと早く飛天を倒していれば… If only I had defeated Hiten much earlier...
俺がもっと早く飛天を倒せば… If I defeat Hiten much more quickly...
ている here describes the continuation of state. See: When is Vている the continuation of action and when is it the continuation of state?
倒す here is a change-in-state verb (a.k.a. a punctual verb, 瞬間動詞)
I have (already) defeated Hiten. (at some point in the past)
The speaker is not thinking about whether or not he will defeat Hiten in front of him now. He is regretting that he did not defeat Hiten somewhere in the past, so ている is necessary here.