That is called confirmation bias. When you already know what your preferred outcome would be, so you structure your methodology to fit that pre-determined outcome.
Let me dumb it down:
Your pre-determined outrcome is: the left is crazy and violent
Your methodology is then: search YouTube (the source of all unbiased information) for examples of the left being crazy and violent
You then formulate (or re affirm) your already predetermined conclusion that: because when you searched for examples of antifa being violent, you found those examples and therefore the left is insane
This is not how you construct a logical argument, buddy
Fuck it's like trying to teach little kids basics of logical thinking.