He's at least an antagonist

You know what, yes. He wasn’t attempting to kill Sonic in order to protect the master emerald, he wanted him dead for petty vengeance. While Sonic occasionally taunts his enemies in order to prevent losing his temper during battle, Knuckles actually enjoys the thought of killing an opponent.

Agreed! Wanting to kill sonic because his father was at war with Longclaw was his motivation

So, is Knuckles gonna get a page here covering his actions in the second film at least? I mean, I'm sure that he is going to be heroic full-time in the next sequel.

Yes, I feel like he was too malicious to be considered a heroic antagonist.