No, the choice at that point is "Let one man go so thousands suffer" or "Make one man suffer so thousands don't". If there was a third choice of "Arrest one man so thousands don't", I would choose that. But since it doesn't, it goes to the defense of others, which is different than murder.
No, it's really the same thing. At that moment, you have only your assumption to operate on, but for all you know, that Tevinter slaver could decide he wants to give up this lousy job and become a merchant in Tevinter. Everything is basically left to chance, and I decided that the chances of him being a better man upon leaving were slim to nil.
So murdering dozens of people so you don't go to jail is preferable? Yeah, you definitely aren't like Howe with putting the lives of many under your ambition in terms of importance.
Talk about false equivalence. The Warden didn't just waltz in there for fun, to steal people's money or simply take the castle for him/herself like Howe did. The Warden is there to rescue the friggin' queen and fight a war against ancient evil that threatens to wipe out the entire nation, and potentially all other nations beyond that. How does that really compare to what Howe was trying to do? Surrendering puts the Warden and possibly Alistair in danger of being executed.
It would have been extremely hilarious if in surrendering, the Warden was executed outright, so you had no choice but to fight if you wanted to live. You are simply metagaming. From the Warden's perspective, you have no reason to believe that you will be shown mercy, or that you will ever be free to finish the fight against the darkspawn, and could very well be waiting to die in a dungeon even while the darkspawn tear through Denerim.
Can't say I'm surprised though. Your avatar has Sera in it and she embraces the philosophy of "It's okay to murder murderers" throughout Inquisition.
As Sera would say: pbfft