Is it justified to destroy or deface property? This is an ongoing debate among anarchists. It's important to realize that the side opposed doesn't seek to force the other side to not destroy property. They merely wish to convince them. In general, there are two general arguments over the destruction of property: Tactical arguments and arguments of unintended consequences.
When it comes to tactics, the side against views it as negative propaganda. Their argument is that people who observe anarchists smashing windows and vandalizing walls will be turned off of anarchism causing them to feel that anarchism is a violent and destructive ideology that causes people to do bad things. In addition, it gives fuel to the media's demonization of anarchism. But those who argue that forget, the typical American who would react that way are not the target audience. Of course some people will react that way, but we aren't talking to them when we smash windows and vandalize courthouses. We are speaking to those who already know the police aren't on their side. The people who recognize that corporate power must be destroyed. They are the dispossessed. They are poor people, especially poor minorities. They already know the enemy because they've experienced it. The police have cracked down on them. The corporations have discriminated against them. They see us and they see someone on their side and they want to join us. That is why we do it. That is the propaganda of it.
Now, when it comes to unintended consequences, opponents argue that the broken windows don't hurt the capitalists. Rather, it hurts the workers who have to clean up or the workers who have to pay taxes to clean up, for state buildings. However, the workers who clean up would have to clean something up or they'd get laid off. Plus, most taxes are income taxes which are far higher for the rich who can afford it. Regardless, these criticisms forget something important: This is only that way because of the state and because of capitalism. The best way to help them is to tear those down. We cannot lose sight of the long term because of the short term. We are helping those workers in the long term, even if they are hurt in the short term. In addition, just as the general public aren't the target audience, the workers who clean it up are. They can see how we are fighting against the system with great harm to ourselves for no apparent immediate benefit. That can inspire.
This discussion wouldn't be complete without mentioning the arguments which aren't being discussed. No one, save ancaps, but I'm speaking of anarchists, are arguing there's something inherently wrong with vandalism. If we smash the windows of a Walmart, we haven't done something wrong, unless there's some consequences of that which are wrong. Property isn't people. It isn't violent to smash a window or spray painting a wall. In addition, we (generally) aren't talking about indiscriminate smashing and destruction. We're talking about destroying corporate property. We're talking about lighting trash on fire to reduce the effects of tear gas. We're talking about defacing public buildings. We're not, generally, talking about destroying mom and pop shops. We're not, generally, talking about destroying people's homes. To say otherwise is disengenuous.