How to share frustration or negative personal experience without being toxic or actually negative?

For example, we have been chatting with friends and one mentioned their unpleasant situation with another friend(tbc) that happened some time ago but still sits in them. I start saying that find that “friend” flake and told about my frustration as well, but then felt like I was unreasonable (even if I knew I was reasonable) and judging and kind of brought some negative vibe. I know it’s not cool to discuss people behind their back and for the sake of truth I told to that person all what I think in their face back then. My goal wasn’t to downplay them or ruin their reputation… ig at that moment my collected frustration just went out when I heard that another person also got bad experience with that questionable friend… but still feels bad, and at the same time what is the point in those chats and talks if not to seek for an emotional support etc. Does anyone else have ever felt this way? How you found your way to manage yourself during this type of conversations?