Thanks for the question Anon! 

But, I don’t think it’s out of character at all. The second half of this season has displayed how frustrated Sam is with the lack of control he’s had in any given situation, and this continues through this episode when he has to watch Jack be manipulated by Lucifer (but that’s a whole other post I’m working on). 

At the end of the episode Dean has already gone through,Gabriel, the only archangel whose shown an ounce of care for humanity has just been killed, he is alone with Lucifer again, and so he reverts back to his Season Four/Five Sam’s ideology. “Forget logic, I’m pissed and I need to do something to be in control of this situation, because, dammit I’m tired of this destiny bullshit, and I’m tired of having my agency taken away from me by this monster.” It was a snap judgement that, at the time, made perfect sense to him. That doesn’t mean he won’t regret it later.