It’s not just some buildings. It’s people’s community. Their businesses, their places of recreation, their community gathering places. That built environment was the labor of many people who left more to their kids than they started with. Property is the difference between Bangladesh and America. Many of these communities never rebound after riots like this. Destroying city blocks like this is profoundly anti-social and damaging to the people who live there.
The difference between Bangladesh and America is a bit more than just property, and the difference between either and some other places in the world is that they are both extremes.
This isn't a case where they attacked him and then he pulled out a pistol and shot them. This was open carry with a rifle. To answer the point you made, they made the decision to attack a clearly armed man and thus evaluated that attacking those cars was worth their own lives.
Any argument that Rittenhouse should not have been there to begin with also similarly applies to the dead. That they instigated the incident is the reason we call it self defense, as compared to the irrelevant school shooter analogy.