This is a painful moment for me. This is the first time that Dean is actually afraid of Sam. Usually we see Dean scared for Sam, like when Sam had the visions, but in this moment, Dean is terrified of Sam. He finally realizes how far Sam went to get his powers. Jensen does an amazing job getting this across with just his facial expressions. I think the moment that really gets me is Dean’s flinch when Sam puts his hand up. Dean thinks the one person he has always trusted is going to hurt him, and he doesn’t recognize his little brother anymore. I hate to say it, but for a moment, all Dean saw was a monster.