Well, it was a building Povey ordinarily had a right to be in until protestors illegally chained all external doors shut. So, calling it "breaking in" is a sorta backward way to think about it, in my opinion. His right to be in the building was curtailed by people who did not have a right to be in the building (or at least did not have a right to exclude access).

As for the university explicitly telling him not to try to enter the building, that may be the case, but keep in mind that his personal work and career were being compromised by an illegal act that had dragged on FAR beyond reasonable accommodation (the sit-in had been ongoing a month and the chaining of doors went on over a week). Just because he was curently a Hopkins employee doesn't mean that his research was owned by Hopkins. Universities aren't like regular companies. Hopkins had a duty to him to provide safe and reasonable accommodations for his work. The grant money he obtained probably had a large payment to the university for these accommodations. Hopkins failed to uphold their end of the bargain with him by pussyfooting around blatantly dangerous and illegal activity by protestors.

What he did was imprudent, maybe even fireable. But unless the students who incited this all are also expelled (I'm pretty sure they weren't), then this would be a significant injustice against the professor due to selective enforcement of policy against some but not all Hopkins members.

Also, seeing Hopkins' sleezy firing letter makes it obvious that there is no good faith on the part of the university. You can tell by how it is written in "you admitted that we had to fire you!" style, that they grossly and intentionally mischaracterized statements he made to appease university lawyers. Dr. Povey doesn't look great, but Hopkins looks like a place where no decent person should ever hope to work.