key word is venting here. Venting to others of your anger does not in anyway imply that you are necessarily bad mouthing someone. That someone legitimately offended us is one thing but where it becomes a problem is when you're saying things that (as I said) you never plan to bring to their attention directly at any point. In other words it is understandable to avoid confrontation initially but if you never plan to remedy it directly with the person themselves at any point, then really you should not be talking about them in that bad light. Also the nature of your "venting" may make a difference too. Talking to another friend or colleague about the behavior of another person with the intent on getting advice and/or help with what to do is also not bashing nor back stabbing but genuinely seeking advice.

Everyone needs a sounding board at times, but what and how you say what you do matters.