A couple days ago I was asked by my former bachelor degree advisor (let's call him X for short) if I were interested in doing a MD thesis with him. I have replied him that I wasn't sure yet about what I would like to do for my MD thesis and I told him I would have asked some other professor before deciding. He said he was perfectly okay with that.
I have therefore talked to several professor and, among them, to an ex collaborator of X, let's call him Y, who works in a different country. In the mail I wrote to Y, I did not put X in cc nor I have mentioned him.
Y replied to me, putting X in cc, and suggesting that I should ask him if he were available to be my internal advisor first.
Finally, X sent a mail to Y, with me and a friend of mine (who wrote the mail to Y with me, as he were interested too) where he said I was trying to "outmaneuver" him by writing directly to Y. He also reported a mail written before I wrote to Y where, among other personal opinions on me, he suggested I was a mental instable person and that I push around my girlfriend (X was the bachelor thesis advisor to my girlfriend as well).
Now, of course the reaction of my professor is beyond any acceptable boundary, but what I really want to know is whether I am wrong at all. Should I have asked him before writing to Y?
Also, how should I reply to X?
Beg your pardon for the Xs and Ys...