Etiquette in difficult situation with colleague A former colleague of mine who I thought that I knew quite well has recently become verbally abusive towards me because I would not leave my current job  to work with him in his lab on different project at another university. 

Looking back on our interactions I now see that he was manipulating me much of the time and was angry that his latest effort was unsuccessful. I am concerned that because I have refused his request he will try and ruin my reputation (he made some veiled threats that I won't go into and implied I owed him because he "mentored" me).

To make matters worse, I had agreed to collaborate on some things with him before his behaviour became obvious to me.  

Is it appropriate to simply avoid/ refuse all contact with this person or will this make things worse (especially with the collaboration dilemma)? 

How would you deal with this? I do not feel that I have done anything wrong and that his reaction was very bizarre.
Threatening is always manipulation. Usually there is talk than walk behind it.

it is not option to conform to his threatening behavior, and you certainly do not want to work with such a person. You owe nothing to that person if verbally abused you. So you should minimize interaction with such a person. Can you pull out from those collaborations?

There might be anti-discrimination institutions that -could- be helpful, depending on circumstances.