Aramis feels guilty for not having been there to help the others in episode 2.
It's not necessarily that he thinks he alone might have prevented the abduction and saved the day when the others couldn't, but his presence there would certainly have helped.

+ for Treville being disappointed in Aramis and letting him know/feel it
+ for the others also making him feel like he's let them down
+1000000 for Aramis angsting

optional happy ending, gen or slash