To add to Irvine, even if you go with the Idea he couldn't shoot Edea because she was basically thier mother (Not exactly, but close enough), what re-assured him was that even if he failed Squall and the others were going to kill her anyway.

...So basically he's a slimy douche bag if you accept that reasoning.

"Wait, so the blood will be on your hands, and not mine? Well theres a weight off my shoulders!"