This was my problem with it, too. He's no better than Anders at that point. He outright admits that he knows Anders isn't in Kirkwall anymore, but he's going to teach 'em a lesson for daring to ever let him set foot in their city anyway. Never mind that the mages there ran him off on their own and no one (except Hawke, depending) wants anything to do with him anymore. Sebastian just throws a fit because he apparently can't track him down on his own.
Edit: To answer your question, OP, I also picked Cullen's option there. it was one of the few times I was actually "Yes! Send in the troops!!!" Like....encourage Aveline to surrender?! Josephine's option I think?....Aveline's only defending her people. Sebastian's development didn't exactly surprise me, considering he kind of did promise to burn Kirkwall to the ground after I spared Anders.
Josephine recommends staying out of it. Leliana is the one who wants to support Sebastian. Which kinda makes sense - Leliana's still in mourning for Justinia, who died at the Conclave meant to end the war that's using Anders' name as a rallying cry.
Still, yeah, I unquestioningly go along with Cullen on this one. Sebastian even acknowledges that a living Anders has long since fled Kirkwall. He just wants to get revenge, go after anyone who may have harbored him. He has learned nothing, he's stuck in the same place he was the day he puts that request on the Chanter's board. 'Someone hurt me, I hurt them back.' He is, effectively, the poster boy for the Chantry. After all, the foundation of their religion is 'mages killed our God's mortal prophet, therefore mages need to be locked up and be treated as subhuman.'