When we criticize antifa violence we criticize violence that attempts to stop racism and racists. Similarly, when we criticize anarchist violence we criticize violence that attempts to stop intolerance; that attempts to disrupt the capitalist system; that is used in self-defence against police brutality; and that ultimately protects rights. By labeling each side as bad as the other we neglect the danger the Alt-Right and these spin-off groups pose.
Antifa is a violence-focused approach to politics that spun off of the Communist Party in the 1930s; various flavors of imitators have been springing up in recent years. As their name implies, they basically try to excuse everything they do as 'anti-fascist' action. In reality, nothing they do actually opposes fascism; fascism is opposed by deliberative politics insisting on reason rather than force, not by smashing windows and burning cars. When we criticize antifa, we criticize the attempt to replace real opposition to fascism with people who think they can engage in low-level domestic terrorism just by calling it 'anti-fascism'. Nobody is fooled. No one is just because of what they claim to oppose, only because of what they themselves do or do not do.
The entire argument is somewhat astounding. 'Group A is bad, very bad, so if you criticize Group B's response to it, you are neglecting the danger that Group A poses.' This is arrant nonsense, deserving of nothing but contemptuous dismissal. Group B may also be bad. Nor is it any kind of defense that Group A is worse than Group B, as McDonnell tries to argue. That a poison is not as horrifying as another poison does not make it more palatable; the only thing that matters is that it is a poison. There is no forced choice here; one does not say, "Oh, since nerve gas is worse than cyanide, I must tolerate cyanide in my diet to avoid dying of nerve gas."
I'll pass on both, and regard people like McDonnell, Antifa apologists, as being contemptible for the same reason I regard Nazi apologists as contemptible: they are poison to all efforts to build a just society.