My impression is they had the goal of altering the outcome of the vote, and clearly some of the group were willing to use violence to achieve that aim.
That is an extra legal activity obviously, and had they somehow succeeded, it would have been a very bad thing in my view. Every subsequent election would be hostage to threat of force. The System would simply fail.
Had they protested outside the Capitol, they would have been within their rights, aside perhaps from not having a permit.