YTA

Here me out, did you do this because you cared about your boyfriend? Yes. Are you responsible for the outcome, kinda.

Here's the thing, you called his therapist and told him EVERYTHING that didn't need to be said. You telling the therapist that he has weapons in his house (sure he told the police that nugget too) as well as about his empty threat to shoot his boss. Your boyfriend is lucky to be alive at this moment because the police would already be on high alert that a potential threat, with firearms lives in that house.

Furthermore, the therapist shouldn't have called his "boss". I don't think you fully understand the full consequences of these actions. Your boyfriend getting fired from his job was bad enough, to now have him labeled as mentally unstable means he could never ever ever use that place as a reference. We don't know why he was fired, he could have simply been let go. He could have failed to do some mundane thing that was just against protocol and they had to terminate him otherwise but still would have qualified him to use them as a reference.

Pile all this on, look at what state your boyfriend was in before and now imagine the state he is in now? He no longer can trust his therapist and if his depression spirals out of control EVEN if you feel you did the right thing your conscious is always going to question it.

YTA because you didn't think this through to what the whole outcome could be. You should have never told his therapist all of these things and instead told him you were worried about his mental state after being fired from his job.