After killing J'ghasta and finding his Black Hand robes, Aerrun realises she's been duped. She has no way to contact Lucien save leaving a note in a prior dead drop, so she decides to try and investigate her false contracts and make contact with the targets to try and figure out exactly what's going on and who's behind it. And maybe even catch up to whoever's replacing the drops in the first place.
Except she's always a step behind, and the other Silencers and Speakers are all convinced that she's there to eliminate them and so they attack, unwilling to listen for fear of being killed, so she's forced to put them down in self-defense. By the time she reaches Ungolim, she's really tired of these other Black Hand members just attacking her outright, so she tries to sneak up and subdue him... but she is a big nord and he is a little bosmer and she's not used to Not killing someone, so he accidentally ends up dead anyway. Cue Lucien showing up in a rage because he's pretty sure she did it on purpose because hey, it looked very intentional, only for her to be like 'babe I swear, I was just trying to talk to him, it's not my fault he's got glass bones.'