"Allowed"? How? He wasn't responsible for the criminal lack of security that day and he told the protestors to "peacefully and patriotically make [their] voices heard." Obviously they didn't listen. Not once did he say or even insinuate anything about entering the Capitol, and without the Capitol being breached, this is a non-event.
As for the gallows, if you've seen it, there's a sign on it that says "this is art" and in no way could that have hanged even Kathy Griffin. Was it in poor taste? Absolutely, but I've never excused any actions like that or breaking into the Capitol.