How a player is doing is no excuse for raging at them, whether it's ranked or normals. That you can admit to having raged at him, however useless he was, only shows that you feel justified by his actions in the game for punishing him as a person.
Sure he was an idiot. But that's no reason to bog down your team by filling up their chat log with negativity, possibly distracting them, and no one deserves to have someone rage at them. Chances are that if he was doing absolutely nothing he was either still learning, which raging at them doesn't help, or he was trolling, which raging at them is what they want.
We've all been there. We've had bad supports. Bad ADCs. Bad mages. Bad tanks. Been a jungler and watched every lane losing ground from stupid mistakes while all three are pinging for ganks at the same time.
Sorry, that last one's a bit vivid for me.
But raging is raging, no matter the cause, and the person on the receiving end still has a beating heart. They aren't the champion they're playing, and they're worth more than their gameplay. I can have similar problems with raging, and can come across as unnecessarily hostile when I'm continuously seeing moments that people are making stupid choices that could have been altogether avoided had they listened to advice, and I'm constantly working on improving myself so that I can refocus that on where it should be.
On minions. Dude they're just asking for it.
Look at them.