Why? Being prepared to kill when necessary is what sets Wonder Woman apart from Superman and Batman who both (ordinarily) have a strict no-kill policy. It's been addressed a number of times in the comic books, but perhaps most significantly in the Maxwell Lord storyline (although that was pretty poorly handled).
As both Superman and Batman kill in the DC cinematic universe, then I fail to see why they would suddenly give Diana a no-kill policy, especially when she's from a nation of Amazon warriors and was shown to be prepared to kill Doomsday in DoJ. Not to mention the fact that, at least in the movies, Wonder Woman openly carries and wields a killing weapon (i.e. her sword/s) which is a sharp contrast to Superman (who carries no weapons) and Batman (who normally carries no particularly lethal weapons, even if his vehicles have weapons).
IF Diana happily slaughters human soldiers left and right in her solo movie, then I'm sure people will take issue. But, that's because Diana is skilled and powerful enough to disable humans without killing them. Of course, given that it is her first journey to man's world, there may be some leeway given that she would be uncertain of her own strength levels, the strength levels of ordinary humans and the capabilities of the many weapons used by human soldiers. If Diana is truly hunting down a god in disguise, then she may also show little restraint if she never knows whether she's fighting a human, a god or a human infused with special abilities by a god.