Being forced to kill can (and often does) cause PTSD, which is why police officers are required to be debriefed, and talk with a psyche professional before returning to a non desk job. Some don't ever want to return, the trauma is so great.
But, the other side of the coin is this: survivors often feel personally responsible for having being unable to prevent death, and can become incapacitated by the guilt, as well.
Either way, a trumatic incident will cause emotional trauma. IMO, better to try to minimize the actual body count than the emotional distress. Not knowing who may be armed & dangerous is the first deterrent. When that fails to deter someone, being able (& willing) to end it quickly is the final deterrent - beyond that point, the shooter will kill no more innocent people.