@Twia
As said before, Belyy is a vegetable IRL. He's allowed to "live" by plugging his physical body to supercomputers and the like, allowing him to exist as an AI - a digitized mind if you will.
Being essentially a supercomputer, he can toy with the facility's electronics as he wishes, which includes the helo and the combat drones (if you notice, the chopper is unmanned, it does not even have a canopy).
If he goes on a rampage only now, it's because this is where he becomes aware that he's not three minds made one, but merely software (with a wetware basis). And being merely software, he's caged to whatever hardware enables him to exist as software.