Who cares? I don't care if some random person I was plotting with blows a gasket and leaves the convo. I also don't care if someone just leaves when things seem to be going okay. Most of the time my conversations over PM are very short, and anyone I'm exclusively talking to through PMs isn't going to be someone I put much value on. I struggle to consider a real friendship contained solely to this website in an age of unlimited communication options. If someone wants to contain me here, then that tells me they aren't very interested in getting to know me beyond the username: Mitsu. It tells me that they're ashamed or embarrassed by the association they have with me through this website, that they don't trust me, or they aren't comfortable with me as a person. That's their prerogative, and that's perfectly okay with me, but I won't consider that person a friend. Whether it's planning a role-play, actually role-playing, or just a casual chat; I don't value PM conversations very highly. Those people are complete strangers that I don't actually care about on any kind of intimate level.
Besides, I don't think I'm so important that I should act with indignation that someone wants to hit the eject button and not talk to me-- for any reason. Despite what this message may lead some to believe, I don't have that kinda ego.
To that effect, if I don't really care about someone, I don't have any qualms with just leaving the conversation on them. I won't feel like I have the need to do so without a concise "good-bye", but if I feel the need to I will. If I'm not vibing with someone and they're annoying me, then I will just skedaddle. It's a lot better than me giving them a tongue lashing over what I dislike about them and why I think everything they're saying is grossly stupid. If I'm at the point where I'm going to just leave a conversation, then that's likely what my next message will be if I don't just Batman my way out of the convo.
I'm not so important or desirable that people will be weeping for weeks if I leave a conversation on them. They'll stop giving a damn within days if not hours. Sometimes human interaction isn't pleasant, and the quicker we learn to process and move on the better. I have zero issues with treating people in a civil or polite manner, but if they're annoying, frustrating, disgusting, or creeping me out then I will grab my ball and go home. It's not worth the mental energy, and I don't let anyone live rent free in my head.