It's no surprise that an alternate ending would involve Marlene getting the jump on Joel first, but man, that's fucking rough. Least Joel had the mercy to kill her quick, whereas Marlene is straight-up torturing the poor bugger. Nasty stuff. While it's still just a 'what if' scenario, if this sort of action from Marlene is within the realm of her character, then it goes to show potentially fucked up Marlene had gotten across her travels. She at least shown plenty of remorse about having to sacrifice Ellie, but torturing Joel is still pretty out there, even for her.
Though I assume this image is based after Joel's slaughter of the Fireflies during his rescue of Ellie, so naturally she's going to be pretty peeved off with the guy.
Anywhoo, ND are going to be talking more about ''What could have been'' with TLoU for PAX Prime, naturally including this alternate ending here, so definitely looking forward to that.