Maybe he was convinced that Taylor Swift was taking over the minds of kids who were going to turn into zombies and take over the world, and he had to stop them and save the world. If he was psychotic, there could have been a million and one thoughts he had that would appear to be completely crazy to a mentally well person. We don’t know, and won’t know until the trial.
My DM has a psychotic condition. She once told me that all the vegetables in the house were going to kill us, and we had to take them to a particular local beauty spot to dispose of them. Luckily she was sectioned that evening, but if she hadn’t been I’m sure she’d have gone to that place to dump the veg, and she would have been capable of getting there. It wouldn’t have been proof that she wasn’t severely mentally unwell.
Also edited to add, I’m not suggesting that he did think that about Taylor Swift. I was just giving an example of the type of beliefs that psychotic individuals can have.