Why do the leftists (and to be fair their right wing is our left wing) always act like this?

I've been giving this some thought as I watched all the rhetoric last night. Even as a former radical leftist, the answer hasn't been clear to me in the past.

I think it boils down the way leftists are primed to think about the world. You believe that oppressive forces control the world. You're a utopian, and you interpret everything through a narrative of victimization. In response to any event, your natural instinct is to create a version of events that reinforces that narrative. Might makes wrong. Any unit of humanity that you could have described as dominant or powerful in recent history is the de facto villain because you can project certain meta-narratives (slavery, segregation, colonialism, imperialism, patriarchy, capitalism, neoliberalism, etc.) on to their role in the event.

Certain entities (America, Western Civilization, conservatives, white people, men) are the most frequent targets of this criticism, and so any time they are involved in an event, even if the so-called "victims" are themselves oppressors (Palestinians, for instance), they must be the bad guy.

This perspective traps the leftist in a pattern of one-step thinking. The reason they think conservatives are racist is because they can project their worldview pattern onto a superficial interpretation of conservative policy. After they've done this, they essentially stop thinking unless they're doing mental work to link this to other examples of "oppression" or further articulate their hegemony conspiracy theory. What they don't do is look at the actual effects of conservative policy.

For example, consider the case of broken-windows policing and stop and frisk. Because the targets are disproportionately black, the leftist has all the evidence they require to demonstrate that something racist is happening. Any other argumentation or evidence thus reinforces racism, and is therefore discarded. Consequently, they ignore the fact that NYC is a safer place for law-abiding black people. This is, in part, why Black Lives Matter fixates over the 100 unarmed black people killed by "racist" police in a year while ignoring the 2,500 black victims murdered by other black people (who should never, under any circumstances, be forced to have a negative interaction with the racist police).

In cases where there's any complexity or ambiguity about the outcome (i.e., Charter Schools), their default interpretation of the outcome is necessarily one that reinforces their favorite meta-narrative mythology (i.e., evil neoliberalism).

tl;dr: leftists have a deeply-rooted, socially-reinforced problem with selection bias.