As someone else pointed out, there's no way for any of us to know exactly what was in KG's head at that moment since none of us are him.
That said.
That was absolutely, POSITIVELY something that he had planned to do before this game ever started. That was absolutely, POSITIVELY a message that KG was sending to Howard and the Magic after all of Howard's elbows. In fact, it was a continuation of what he'd started from before when he shoved Howard and got T'd up in game 4.
KG can seem to get carried away at times, but look when it happened. 4 minutes into the game? He had absolutely come into the game planning to take advantage of the first chance he got to get physical with Dwight. And frankly, the way he did it was just about the most controlled, least risky way to do it. On a break-away the risk of injury on either side and/or flagrant 2 calls sky-rockets. The NBA has shown consistently that the "accidental" elbows among players can lead to suspensions. But I've never seen the NBA suspend for aiming at and connecting with an opponent's arms.
Someone had to throw the bean-ball at Dwight. And really, it had to be KG. Perk is 1 T away from a suspension. Baby and Sheed were both hurt. Short of putting Shelden in as a hatchet man, which would have been pretty obvious, there was nobody else to send the message early.
And plus, in this series, KG's role wasn't to be "KG". He wasn't called upon as a key scorer or offensive focal point. His job was to remove Lewis from the game (making the Magic play 4-on-5 on offense) and anchor the D. So if ever there was a time when KG could afford to be an enforcer, this was it. Lewis doesn't draw fouls anyway, so the 2nd foul (even in fore-sight) was likely considered worth the risk.
Right or wrong, that was definitely a calculated move by Garnett. That was the closest he could come to clothes-lining Howard that the current NBA would allow. And I disagree that the message wasn't received...the Celtics bullied the Magic off the court in game 6. They were the tougher team. KG's chops set the tone, both the Celtics and the Magic got the message, and the Celtics took care of business. On to LA.