This is one of those moments in S8 that just thoroughly hurt knowing what’s going to happen to Dean over the course of the following seasons, because in the end Dean chose to take on the mark himself too. Of course he didn’t choose to become a demon, but he didn’t want to hear about the consequences that Cain wanted to tell him about. He wasn’t in a mindeset mid S9 (depressed, suicidal) to read the whole description so to speak, but only looked at the bigger picture like Dean did a few times over S9 before he took on the mark without fully knowing what it would entail.
I found this moment really tragic on re-watch as both of what Dean says here fits himself a season later. He chose to take on the mark (much more as a form of sentence or to do penance for what he considered he should do due to his mistakes, it wasn’t sparked by any wish for power - but “calm” - in the end it really was something Dean did and took on because he was perfectly alright with not walking out of a fight against Abaddon alive which imo is partly why he didn’t want to hear the warning label) and didn’t want to hear about the bad possible things coming with it for him personally as he didn’t care about what would happen to him and therefore was forced to become what it turned him into. Something he never wanted to be and when realizing what the mark did also rather wanted to die. Ugh, it is horribly ironic in a way that Dean took on the mark in kamikaze mode, so depressed that dying in the fight would have been welcome and instead he was cursed to become a demon and immortal. Man… It’s really horrible. But I just loved the MoC arc so much. It was painful and tragic, but I loved it. And Jensen certainly rocked it acting wise.