He's clearly suffering from a severe mental health issue.
It's astounding that he got to a position of power in the first place obviously, but now his delusions are absolutely off the scale.
He genuinely believes that the election results were a conspiracy, very publicly announced that it was a landslide victory in his favour when it so blatantly wasn't.
He obviously has a long history of utter lunacy, but reading his tweets and seeing recent events makes it clear that he's mentally unwell and a danger to himself and others.