I agree that Josh didn't deserve either of the endings he got. He also never meant for anyone to be hurt. When him and Sam were chased through the basement by Chris, Josh was fully ready to protect Sam with his life until he realized it was just his buddy pulling a joke. And then again when Mike mentioned that Jess was dead. You can see the genuine surprise on his face. Josh is just ill and needed help. He wanted to pull a prank in revenge but didn't want anyone to be hurt.
Dean accepting the Mark of Cain (and losing himself
in the feelings that were washing over him when he was
killing with the first blade) has never been just a product
of the events of season 9, but everything he has been
through from the moment his father shoved Sam into
his arms telling him "Take your brother outside as fast
as you can. Now Dean, go!". All of the self-loathing, the
guilt, the grief, the depression and suicidal tendencies
- all of those things paired with Dean never having
worked through his issues, but instead always
suppressing them, culminated in him taking on the mark.
As an act of redemption, a wish to escape, as a means
to an end. Yes, all those scars - from losing his loved
ones over and over again - have never healed, but kept
hurting, festering. For years. Until he found something
that took away the pain...
Quote taken from 9x19 "Alex Annie Alexis Ann"
"Remix Gifset" (x)