It's not justifiable because we need the institution. It's the same reason it's wrong to kill the president. It's not that it's totally wrong to kill a bad president- it has a lot of good points going for it- but it's wrong to attack the institution of the government. There are plenty of cops, politicians, etc. that have done enough harm to deserve death. One bad mayor does far more damage to society than one cold-blooded murder. Why shouldn't he die? Well, there isn't a good argument for him personally. He does deserve to die. But we don't deserve to destroy our institutions. We don't deserve lynch mobs and assassins and chaos. We deserve an ordered and just society that recognizes institutions that we all benefit from. In other words, you shouldn't kill cops because it's uncivilized.