Because if you start justifying these kinds of things, they will eventually be turned against you and next you know it, your own political representative is being shot at because their opponent's supporters are feeling threatened just as you are (whether they are truly justified in feeling threatened or not, doesn't matter).

Democracy does not work with violence.

EDIT: I'm really curious about the downvoters: do you really think murdering your political opponents is justified? The goal is to vote out the fascists, not become fascists.