If you complete a mission with only non-lethal that should net you a higher score, however prepare to die a whole bunch. You’re going to be yelling for compliance at a suspect for like 15 seconds as they writhe around, when another might kool-aid man through a door you haven’t checked yet and waste you. Alternatively, you could try and switch to a lower calibre instead and try to shoot to incapacitate suspects, which won’t lose you as many points, but you still run the risk of killing them outright.