I'm asking about the manga All Might learns from the villain who has stolen hundreds - thousands of quirks to become what he is that Izuku is somehow related to the previous All Might. Why is that so soul crushing to him?
Why wouldn't it be? The bad guy just explained that he has robbed a ton of people from their quirks (quite possibly killing them in the process) and now when All Might himself is at his weakest, and Midoriya has still not mastered his gifted quirk, the danger is tremendous. Either of them or even both could be attacked and their quirk stolen, which would definitely be extremely bad.