Yeah, I'm sure you would be able to kill a person you loved or cared for without even flinching. Sure, Stellar did horrible things, but she did so without realizing what she was doing as she was being abused by the EAF. She had the mentality of a kid and acted out of pure fear, and Neo going "THE SCARY THINGS ARE COMING TO EAT YOU!!" did not help much. Shinn knew that and wanted to protect her, and I can't really consider him 'a lunatic' for wanting to do that. He made a promise and kept it until the end, as good as he could.

I would have done the same, as would most of you had, I reckon. And that is what I like about Shinn. He makes a much more believable character than Kira.