I was almost expecting Nick to put a “possessive jerk spin” on Donnie in this moment. Like have him flash some sort of smug grin over her shoulder because his “rival” has been put in harm’s way. And I am so glad that they didn’t. That they showed how much of a selfless person he can be.
Donnie cares too much about April to be selfish enough to be happy that Casey was possibly hurt. He knows that she cares about Casey, so he cares about him too to a degree. I just really love this moment because of that. Donnie is perfectly capable to put his resent for Casey aside for April and her well being. So no, I don’t think that he hates him, or is overly possessive, or only does things for April for his own benefit (getting her to like him), etc. He truly cares about her.
Donnie is a total sweetheart and no one can tell me otherwise.