He's only killed once (Gale), and that was to save his own skin. He'd plotted to kill the dealer in season 1 and Gus, and those were also to save his own life. Walt had to cajole and plead quite strenuously in each case, and Jesse only aquiessed when he realized there was no alternative. Each time, he was petrified and it obviously disturbed him tremendously. I think he reflects how most of us would feel, and perhaps act, in like situations. The only exception was the two street thugs at the end of season 3, but the extreme trauma he'd suffered regarding Jane and then Thomas made him snap. He was hardly thinking clearly, and he certainly wasn't himself.