I am an incoming PhD student. Before I applied, I chose a particular school because it had a specific PI. (Of course, this is not a good idea, but I did not know better at the time.) The PI is very famous in my field and was extremely excited about having me in his lab, going above and beyond to get me to work in his lab (e.g. private tours, etc.). I believe he was partially responsible for getting me into this school, but I can never be certain.
I had to take a year off for medical reasons, but for the last year and a half, we have been in monthly discussions about what I will work on and what I will do, and he has been giving me lots of work to do before I arrive. Up until just recently, it was obvious to both of us that when I came to the school I would be in his lab. He expects me to skip my lab rotations and join his lab immediately.
However, I recently learned from a conversation with one of his postdocs that he has being emotionally abusive to one of his graduate students (e.g. comments along the lines of "you know everybody hates you"). I also learned that he has been completely ignoring one of his postdocs (as a result of which the postdoc hasn't published in the last year and a half), and that he gave away the project of his other graduate student to someone not in the lab, and then lied about it.
So obviously I'm not joining this lab. What is the best way to break off this relationship? I am concerned about him retaliating, as he has a lot of power in the department. I also cannot share my true reasons for leaving with him, because then he would retaliate against the postdoc who told me.
Is this something that should be done via email? Skype? I saw him recently when I visited the school (and learned this information) so I did not bring it up then. There are many other labs in the department so I'm not concerned about that. But I am concerned about what the consequences might be for leaving his lab. Also, what kind of excuse should I use? My present excuse is "I am questioning my interest in sub-sub-subfield X and want to do rotations before I start working with you."