Saying an engineer shouldn't design a better non-lethal weapon is like saying a doctor shouldn't treat a wounded soldier.
FTFY (Even that is arguable given they could be used to enforce a police state, and that non-lethal weapons can cause permanent injury)
However of course this may be impossible or impractical with current technology. Ex. I imagine it would be impractical with current technology to disable an aircraft or submarine without virtually guaranteeing the death of some/all occupants.
Wars are frequently started not for moral reasons but merely justified by citing some moral argument with no connection to reality. Designing better lethal weapons for a country (read America) that is already generations of military tech ahead of all allies and much more so enemies with the justification of "saving lives" is an exercise in cognitive dissonance.