Rumple's sacrifice in 3x11 was heroic but not completely selfless. He was seeking his son's forgiveness, and I think he also wanted revenge on Pan. It can also be argued that killing someone is selfish even if it's to protect/save someone (for example, Emma's heart darkened a bit when she killed Cruella to save Henry). This time Rumple's not seeking revenge or anyone's forgiveness (And maybe he doesn't even have to kill anyone else in the process (at least not directly)). He'd do it simply because it's the right thing to do, believing that he will never be reunited with Belle because he didn't find the Guardian (In a way it would parallel the sacrifice he made for Alice).. Only to find out that he was wrong and he was the Guardian all along.
Rumple is one of the most significant characters of OUAT and he's been in the show since the first episode. His redemption arc is the longest. When he exits the show, he'll definitely do it with style, especially if he's going to die. It's going to be huge. And so emotional..
