"... and you're going to burn in hell
for an eternity"
I never heard him say anything like that in his speeches.
Maybe I have a short memory, unless you mean the short NO answers he gives to people that ask whether bending the rules a little is OK to do or maybe you're referring to how he hasn't given up on correcting people about the GNU/Linux naming. I also heard multiple times about how he holds the FSF members to these higher standards.
But have you ever asked a Christian if you'll go to hell for being an atheist? The answer is pretty predictable. And compared to Christianity, a faith based on non-falsifiable beliefs, Stalman was proven right again and again.
Maybe he just got tired of repeating everything like a broken record? Maybe, just maybe, he worked for his whole life trying to build the GNU operating system without the results he wanted? And to put salt to the injury, OS X built on top of those tools. Wouldn't you be pretty pissed about it?
He's not young anymore, he may not have the same patience or pragmatism of 20 year old. He does sound like an old grump. His idealism is kind of obnoxious.
But regardless of that, he's one of those people that pushes the human race forward. It's OK, you don't have to be friends with him to give him a little respect.