I think it comes from:
1. A natural aversion from killing someone. If we could have phasers on stun, would you rather use that?
2. The mantra of why didn't you shoot him or her in the knee, wound them, etc. Another suggestion to disable rather than use lethal force.
I was taking my CHL test once and shot it well (yeah, right the TX test isn't William Tell level). Some guy tells me I should miss some shots because if I go to court, they would say I should have shot him in the knee. This guy knew because he was a 'sniper'.