1. He's a giant hypocrite.
2. He's super judgmental.
(TW: the above links lead to images/gifs of physically/emotionally abusive behavior)
Dean attacks them physically with no provocation, even using it to make sure they behave the way he wants. He puts them down, constantly reminds them of their mistakes, and never allows them to react to bad things he's done.
Speaking of treating someone terribly...
4. He's used ableist language to put down Castiel.
(TW: the above link leads to images/gifs of ableist language)
So, your best friend loses the ability to help you in a crisis. Do you start insulting them and finding different ways to call them useless?
If you're answer is no, then you're a great friend. Honestly keep doing you.
Otherwise, you must be Dean Winchester and in that case you're a jerk. Castiel is more than his angelic abilities to help his friends. But you wouldn't know it by the way Dean insults him and even goes so far as to call him a "baby in a trenchcoat."
But Castiel isn't the only one being reduced to what he can do for Dean.