The motivation to share information is a very important aspect of this discussion as it relates to your example and situation, Lilo. Anyone who truly knows you also knows that your concern for the hearts of others is extremely deep and, in my experience, without equal. You have a genuine concern for all people and especially the friends in our relatively small community. You chose to talk to our friends out of concern for their well being and because you had a personal experience that may have been a benefit to the expectations of this new relationship. There is no harm in that but there is a window for mis-interpretation as to what you wanted to achieve....helping others. I'm sorry this came back to cause you pain but am also sure our friends will eventually understand and things will heal.
Now for my empirical response to the idea of sharing information on ex partners
Unless there is a very real and imminent threat with the people involved, I believe it is everyone's right to enter a new relationship with a clean slate. A history of unsafe sex, dangerous drug abuse practices, physical violence or clear emotional abuse would be examples.
We do not know what has changed for ex partners; how they have grown etc. There is also the very real possibility that your own future interests will hear about you sharing your experiences with other partners which could lead to a pre-mature lack of trust.
Simply stating very casually that you have dated this person in the past is innocent enough. If there is a desire to hear more than you will probably be asked. To offer your opinion upfront (primarily negative) can be seen as a warning which will probably serve to create animosity between you and the ex. It may also be seen as jealousy or vindictiveness.
In the military we pass on the positive attributes of a person but uless there is substantial reason we keep the negatives sealed and to ourselves unless there is a problem that requires a look into their history.
Openning the door to your experiences is fine in my book. Pulling people into that doorway without being asked is a potentially damaging action; one that could hurt you and the other people involved.