What are we calling the illegal part? Feet planted, arms tucked, his gloves are up in his own chest, he delivers his contact straight through the center of his opponent (at least at first) which you can see because Carlo's arm is directly impacted, followed by his shoulder and chest, and then Wilson rolls through the contact on the follow through and gets the gloves to the head.
Somewhere in all that, mind you, he also takes Carlo's butt end to his own face so asserting that he's 100% in control of all of those actions is questionable.
Listen, I don't like seeing anyone's head thunk off the glass either, but it's something that happens sometimes and the conversation that follows regarding minimizing it has to pay attention to both skaters. This isn't a Capitals incident either, I was just saying this in the Mackinnon thread that for as long as players reach in the neutral zone and try to push the envelope of what they can get away with without being hit, these injuries will happen because it's a contact sport.