I once worked for about eighteen months with a peer I couldn’t trust. It was awful. I wasted time, effort and attention on parsing his emails, protecting my team and laboriously documenting “agreements” that “clarified the situation” to my boss (which would be run over an hour after we had met to “ratify” them).
And I felt bad. All the time. There’s just something nasty about not knowing: not knowing what will happen in the next meeting; not knowing if an agreement will be kept; not knowing when another email will come in from your boss, from your team, from a peer, with another set of bruises and breakages and emotional upset. And there’s something more than that: having to be around somebody who you don’t fundamentally trust is spooky, uncomfortable, unpleasant.