In 4x4 Dean's reaction to Sam drinking demon blood is only disproportionately harsh when you forget that he just spent 10 years torturing souls in Hell. He *knows* exactly how manipulative and evil demons are, and he's terrified his little brother will give in to them in the same way he feels like he did.
On another note, I love how in this episode Sam spends so much time being all emo and angsty like "Dean you don't know the darkness of my soul" and meanwhile Dean's waking up from nightmares every night where he's reliving 80 different ways to filet a human body. Dean's just like "Yeah okay MCR" 🙄