I think Seb could.
Would he? Probably not. He's prone to agonizing over things, for sure, especially when he values them. I do have a hard time imagining Sebastian attacking a city full of people who aren't Anders, with his subjects, assuming he pulls off the whole coup without a protagonist in his back pocket (which occasionally happens!).I agree, I think that his reaction is driven by anger and shock. Which is part of the reason why Sebastian is one of my favorite companions, all his warts included. He has some very natural reactions, instead of perfectly evaluating everything.
While Sebastian does most of the things we see with Hawke's help, if you do some codex hunting, the boy is actually building up a powerbase that doesn't sound worthless... Up until he runs smackdab into the "Why do I really want this?" issue. Course, from Hawke's perspective, everyone needs her help all the time. But the characters do do stuff in the intervening years, and Sebastian, even if rivaled, sticks around Kirkwall for a couple reasons. He says the most pressing one is protecting the Grand Cleric, and Hawke if you're besties.
He's totally going to try to kill Anders, though. I'll eat my hat if even the shiniest happiest most diplomatized Sebastian would not try to put an arrow in his skull at least once.
As for blood... Psh, he's had worse. I saw his rivalry power! XD