He went knees first because that is how you slide. No doubt in my mind he wanted to hit that contest as hard as he could, and rightly so. That's football and there is no rule against it, as long as the player does not get hit high or in the back. You may not like it being done to your player, but that doesn't make him a sniper. And if Goodes, Bolton or Jack had of done the same thing you would be applauding.
Lol. He wasn't hitting any contest hard. He ran past the ball to drop the knees into another player who was on the ground. If a Swans player dodged a chance to clear the ball out so they could try and injure a player lying on the ground I sure as shit wouldn't be applauding.
Edit: Hell, looking at it again he doesn't even really slide. The first point of contact his left knee makes is the side of O'Keefe.