Brooks was an extremist who regularly posted that “Hitler was right about the Jews”

This has everything to do with the parent comment’s insinuation that political extremism is only a one way street.

Also you conveniently left out the part where he murdered 6 people.

“He drove into a parade some time later.” Unreal.

So he was a right wing extremist who murdered several innocent people. The parent was saying that left wing extremist attacks aren't nearly as prolific.

Edit: on further thought, no motive for the attack was established. This seems more like an act of random violence fueled by rage, rather than an ideological attack.