They hospitalised a security guard, spath, and set off a smoke grenade in a crowded building. That is paramilitary tactics and is not acceptable.
For the record, I don't think Benjamin should have been invited (or indeed accepted by the university) because he has never contributed to knowledge through research or publication - he's just a rabble rouser with a popular youtube account and a slightly cultish following. But this kind of violence and aggression really needs to be stamped out. This is a serious escalation on earlier Antifa activities at King's and there needs to be a criminal investigation and (if students) expulsions. The university has taken action like this before against radical Islamists at the university, the same principle applies here.
See this as an example of what the same people were doing 2 years ago, in the same building:
http://www.independent.co.uk/student/news/king-s-college-london-launches-urgent-investigation-after-pro-israel-event-attacked-by-pro-palestine-a6825116.html