How do you distinguish between backbiting against warning a person of someone's bad character? For eg:
1. We entrusted someone with money and that person misappropriated it. Do we not tell other people of this so that they would be spared of the same by this person? Do we tell of this behavior to the general public or on an individual basis?
2. Another example would be if we know a person to be of unsound character (bad tempered, disrespectful, violent etc), is it not our duty to tell if someone wants to choose him/her as their life partner?

Only in situations were you’re asked about this person for marriage or business for eg you’re allowed to tell them these issues. Otherwise exception is if you have been wronged clearly and oppressed and it has not been solved then you can raise the issue with people