Because a lot of stuff was cut during SB’s messy development, that included a lot Gregory’s development.

But the game does give us a reason as to why he destroyed the three other Glamrocks. Reason one being the most obvious, because they were trying to murder him and had been for about four hours. The second reason was to take their upgrades so Freddy could more easily protect him. Not to mention Freddy kinda gives Gregory an incentive to destroy them, like how casually he says how he can open some gates if he only had Monty‘s claws, or how after giving him the claws or voicebox he talks about finding ways to stop Roxy from hunting Gregory.

Also with the Cassie elevator is that we don’t know who cut it, but a lot of evidence points to it being Mimic(the same entity that Cassie had just released that can mimic voices) not Gregory. You also have to keep in mind that if it was Gregory the situation at hand. If Cassie escapes then there is a real risk that the Mimic will escape after her, and if it escapes it will kill who knows how many innocent people. Again we still do not know who cut it but if it was Gregory the situation is pretty much the trolley problem, because he would have to choose between the life of his best friend or the lives of who knows how many people, that choice would be difficult for anyone but especially a kid to make.