I would say unbind your chat button. If that's even a possibility of a thought in your mind, it means that you are not in the proper state of mind all the time to control what you are about to say.
That being said, short and abrupt phrases tend to put people off a lot easier than saying something with a positive spin on it while trying to correct a behavior. On the phrases you mentioned, I could easily see someone on the other side of the monitor pounding on their desk or waving their finger at the screen while they are saying them to me. Plus, I don't think I've ever typed in chat "Please at least give us a 'Gotta go' before bailing entirely." when I heard the "Summoner has disconnected" chime. That phrase in itself is negative just because you're only piling onto the mentality of the 3 teammates that you have left that the end is near when the summoner might have just been having a client issue and will be back as soon as he reboots. Saying things more like "We weren't all there, let's fight 5v5" or "Let's meet up at mid lane" or "That sucks about your connection, has it been bad all day?" are far more "positive" and will lead more to the team thinking you care so much as you just being a jackwagon and scolding them. Even if you don't care about someone on an individual level, you are in effect USING that person to climb the ladder. You might as well encourage them to play better, even if it's just for you in that one game.