How to inform an emotionally abusive advisor I don't want to work with him?
My answer assumes that you are convinced that your presumptive advisor is emotionally abusive, and that you have made a firm decision not to work with him.

However, you haven't given a first- or even second-hand account of abuse or neglect; it's also possible that the postdoc is blowing a personality conflict out of proportion by spreading false accusations.  The only direct red flag I see is your presumptive advisor's expectation that you will join his lab directly without doing rotations.  You may want to have a confidential discussion with another senior faculty member about your concerns, before jumping off this particular cliff.



Pretend that your presumptive advisor is a reasonable adult human being.  Talk to them in person.  Be straightforward and honest.  


  "Thank you for your encouragement and your support, but I have decided not to join your lab."


That's it.  You do not owe them an excuse, an explanation, an apology, or a description of your alternate plans.  (I think you do owe them thanks.)

If you would be more comfortable giving an explanation, then do so respectfully, honestly, and carefully.  But if you have no intention of working with this advisor in the future, do not suggest the opposite, even indirectly.  In particular, you should not say "I want to do rotations before joining your lab" unless you actually intend to join their lab after doing rotations.  It would be better to say something like


  "I have decided to do full rotations before joining any particular lab.  I want to explore a few different subareas, and I think the breadth of experience will help my long-term academic career."


Be ready for the advisor to pressure you to do a rotation in their lab, or to argue that you're actually damaging your long-term career by not joining a lab directly.  Thank him again for his advice and encouragement, but stick to your guns.  Or he might surprise you by being completely supportive and giving you good advice about other labs to work in.

Under different circumstances, I might suggest being direct about your suspicions of emotional abuse and neglect, but as you say yourself, that could backfire on the advisor's other students and postdocs.  That's their fight to have, not yours.

If the advisor is as emotionally abusive as you suspect, he may try to pull you into an argument or blackmail you.  He may say things like "I got you admitted to this department; you owe me." or "I'll have you kicked out of the program." or "I'm big and famous and important, and I know everybody, and you've just signed your academic death warrant."  Do not engage.  Do not argue.  Do not back down.  You have neither signed a contract nor sworn an oath of fealty.  The most you owe him is your sincere thanks and your future success, on your own terms.  Assuming you work in a healthy department, his threats are idle.

That said, you should have someone in your corner, just in case his threats are not idle.  Before you dump your advisor, I strongly recommend having an honest and confidential discussion with the graduate director, ombudsman, department chair, or another trusted senior faculty member.  (The other students will know who to talk to.)  Be completely straightforward about your concerns, especially about possible repercussions from your decision.  Hopefully they will reassure you that your concerns are groundless, or at least that someone else is willing to fight for you to be treated fairly.

If not, you're better off in a different department anyway.