When someone figures out Daredevils identity and threatens to expose it unless they're paid for their silence, Foggy, in a desperate attempt to protect Matt, ends up killing them because there's no possible way for him to find the money to pay. He doesn't intend to do it, it's just the way things work out (maybe negotiations go sideways). He's fairly sure he won't get caught - he's dealt with law enforcement long enough to know how to clean up a crime scene - but he still has to deal with the fact that he was willing to go so far (or further) to protect his best friend.