Sure, he openly is seeking revenge again even though the Grand Cleric says Death is not Justice. But the spirit of Justice became Vengeance and rained death on Kirkwall, so Justice is really screwed up. The Grand Cleric was like a mother to him. Can anyone here who grew up with parent-like figure say they always agreed with that person's concepts? Anyone? I can't. And sometimes that means I've gotten burned but it doesn't mean I didn't act rashly and did what I want instead of what was taught to me.
Some people don't like Sebastian because they're bothered by the fact he's religious. It's like the people who dislike Fenris because they hate that he hates mages. But you know what? Sebastian DOES NOT betray you. Sebastian does not lie to you. Sebastian is relatively sheltered, only 28, and he's lost his entire family, he doesn't really know if he can handle being a king, and he admits it. He feels a responsibility, but he's worried he wants power for the wrong reasons. He's honest and open about who he is and his fears. You can feel free to dislike him, but of course his talk is going to be preachy. He's been in the chantry for his adult life. He went in when he was THIRTEEN. The man hasn't really experienced a different life.
The more believable last scene would have been for Sebastian to have picked up his bow and someone have to tackle him before he shot Anders himself. Because honestly? That's what I would do. If someone just destroyed my home in front of me and killed my mother figure in front of me and said they did it JUST SO THERE COULD BE NO COMPROMISE?!? If I had something remotely weapon shaped near me, I'd be after them. I'd let Hawke deal with the battle with whatever side Hawke chose, and while Anders is sitting there pensively, he'd be dead.
As I said in a different thread, we also don't know if Sebastian won't reconsider things once he's had a chance to grieve after he's left. He has a lot to handle in getting his throne back and making sure things are truly under control in Starkhaven. But give the guy a break, okay? HAWKE was the LAST thing Sebastian had that was familiar. Absolutely last thing... well, besides his belief system. Which you also see him rely to when he chokes back his grief to commend their souls to the maker. Why wouldn't he look to you to take care of the issue? I think it's quite understandable that he looks to the very last thing that's familiar to him. Starkhaven isn't familiar anymore, his home is gone, his family is gone. All he has is the people he's become friends with. And Sebastian discusses with Fenris how they should be turning in the apostates, and Sebastian even nervously tries to suggest lots to decide whether it's him or Fenris. Fenris, in one of the more amusing moments of the game, goes "Nuh huh. If you want to turn in one of Hawke's friends, you take it up with Hawke." Sebastian never DOES take it up with you. And Fenris hits the nail on the head earlier in that banter that Sebastian is only hesitating because of Hawke. Now one of those apostates which Sebastian felt should have been turned in...just destroyed everything. So yes, he shouts angrily in his grief and makes a threat and is hurting that Hawke betrayed him by standing by Anders. And you are betraying him. It's not like Anders denied doing it. As I asked in another thread- Hypothetically, if, for some reason, Varric had been in the Chantry when Anders blew it up and Varric was dead, would you still be so quick to say Sebastian is whining and how dare he think Anders should die? If it was one of the companions like Varric that most people REALLY love? I think people would be falling on the other side of the issue in that case.