Yes. It was a cruel thing to say. But! Loki was not at his best at the moment (probably at his current worse) he basically was in the middle of an identity crisis, trying to prove himself to himself, and wanting Thor to stay the hell away from Asgard and yes, his choice of words were hard and cruel, but bear in mind that he was the antagonist in this movie and the audience has to side with Thor. Best choice? Make the audience dislike Loki.

He being mean and cruel was the easier way to accomplish that. But then again, It helped Thor to try and settle in what would have be his mortal life and then understand something was pretty fucked up with his brother, to the point of the fight and then Thor making the sacrifice, proving himself worthy and whoohoo, he's back.

So, basically the movie did exactly what the writers wanted: your sympathy for Loki was gone, because, hey! He's the bad guy; you're not supposed to like him, right?

Loki is not a dislikable character, but he has to be for ¾ of the film, because he's the bad guy, so he has to be horrible to the good guy, so you will dislike him at least while the climax is in progress, otherwise it wouldn't make sense. This is not a book; this is an action movie, so resolutions have to be that way.