He was supposedly assassinated by his wife Catherine II and as a result, the history books really don’t have many nice things to say about him. All descriptions are mainly drawn from his wife’s memoirs. Her words to describe him include: idiot, drunkard, good-for-nothing, hideous, loathsome, of bad taste, coward, wife-beater, etc. (clearly, she didn’t beat around the bush.)
Catherine took over the throne in just six months into his reign, and had Peter declared as “crazy.” But crazy is as crazy does- along with a couple other people who could have potentially got in her way, Peter was found dead surrounded by a cloud of mystery and suspicion.