Likewise known Proud Boys and 3%'ers were present, etc... This was the Place to Be for all militant right wingers, and those who couldn't be in DC were staging protests elsewhere. IIRC there were violent actions on the same day at both the Oregon and Kansas state houses.
Everyone Knew they were going to be there. Everyone Knew they had the potential to be violent. And Everyone Knew that Trump's rhetoric was inciteful. None of this was a surprise. And it finally boiled over into real violence. There's no excusing or downplaying that. We could absolutely have seen this coming, and lots of people did.