This is the reality society refuses to acknowledge. It's ugly, it's violent, and it's necessary. Use all the Sheepdog/Wolf references you like, the reality is that, sometimes, violence is the answer. This applies to the law enforcement officer as well as the civilian.
Your willingness to do violence upon another when necessary should be established before you ever take on the profession or purchase a weapon. Once established, your ability to do so should be honed to as fine a skill as your abilities allow. Anything else is unacceptable and inexcusable. Our communities and families warrant and depend upon it.
While there is no shame in not possessing the willingness to do this, there is however in not being able to admit it.