Illidan didn't sacrifice himself. The Titans outright told everyone to leave because they could contain Sargeras all by themselves. Illidan chose to stay, and not out of any necessity, just because he wanted to. He could've returned and made ammends but in his own selfish way he chose to remain at the Seat of the Pantheon and watch over Sargeras to satisfy his own ego. It's not like he even has a chance against him if he escapes the Titans' prison, he is just there to mock him. It's probably going to get old pretty fast and will become pretty awkward for everyone there once he runs out of things to say to him.