At the individual level, of course, it is always possible — even likely if it’s a large enough crowd — that a violent crowd includes some number of people who are looking to discredit the cause being protested for. It is also true, in nearly every case, that any violent crowd is only a subset of a larger, peaceful crowd protesting for the same thing. Neither of these facts matters — not Wednesday, not in the case of this spring and summer’s riots, and not in other cases of mob violence. The obvious, logical inference, which experience regularly bears out, is that the bulk of any violent mob are who they say they are, which in this case means Trump loyalists. That seems more obviously true here, as a number of the rioters at the Capitol cheerfully gave out their names to reporters, and can be easily checked out. Large numbers of arrests would help, as they helped this summer, in pinning down who was behind the mayhem. And there should be such arrests: anyone who breached the Capitol ought to spend time in federal prison.
Another theme going around at the opposite end of the spectrum is that the rioters were white supremacists. Now, white supremacists tend to be the most extreme of extremely political people and quicker to violence and lawbreaking, so undoubtedly they were more represented among the rioters than among the general run of peaceable protesters on Wednesday. Some of the known white supremacist groups were clearly active in organizing ahead of this. But fundamentally, the presence of white supremacists is neither here nor there. Because the “Stop the Steal” rallies were not about white supremacy, the problem they present is in no way limited to white supremacists, and is not entirely driven by them, either. It is simply a cause to which some of them have attached themselves, and focusing on their involvement risks misdiagnosing the nature of the problem.