Kurt considers not throwing the note.
It's not that he considers continuing to argue that he should perform Defying Gravity at Sectionals – he gave up that aspiration as soon as he saw his Dad's expression after the phone call. But butchering the high F wasn't the only way to accomplish that.
He could have simply withdrawn his desire to compete, and declined to give an explanation. Some of the club would have questioned him – well, Mercedes would have, at least – but Mr. Schuester wouldn't risk an easy solution to a problem he clearly didn't care about in the first place. Or he could make up a reason, like he wants his solo performance debut to be a different song, or even something as mundane as stage fright.
He could even go through with the diva-off properly, and if Rachel wins (which, bravado aside, he knows is a definite possibility – she does have a better-than-good voice, as much as he'll never admit it out loud, and even within glee club preconceived gender roles have the odds stacked against him) he wouldn't have to say anything. If he won, he could still decline to perform.
That option is tempting. He dearly wants to prove that he's just as capable as Rachel as doing justice to this song.
Or, he could be truthful about why he won't perform. He could make everyone understand, at least a little bit and for however briefly, what being out in a town like Lima is like. Remind them that no matter how much they ignore everyday homophobia, he can't. (And yes, he is thinking specifically about Mr. Schuester. He plans on being bitter about how that man walked passed him getting thrown in dumpsters for the rest of his high school career, at least.) He could make sure everyone knew that he's taking the moral high ground, that he's not as willing to throw anyone and everyone under the bus for a solo as Rachel is.
He doesn't, in the end. He didn't come out that long ago, regardless of how many had assumed correctly beforehand, and he's not quite ready to get up on a soapbox about homophobia and bullying. Besides, school has long since made him far too cynical to believe it would actually change anything. Oh, he can daydream about making them feel guilty about all the times they threw him in a dumpster, or slammed him into a locker, or hurled a slushie at him, or insulted him, or tossed pee balloons at him, or stood by while other people did those things. But if he really thought he could say something – anything – to make people feel bad about targeting him, or get a teacher to intervene on his behalf, he would have done it ages ago. The last thing he needs is to let his probably-unrepentant tormentors and their enablers know that they've managed to hurt him.
So he takes the route that causes the least attention, the least conversation, and throws the note.
It's horrible when he does it. He regrets it, a little, when he sees so much of the club looking so unsurprised, even smug, that he blew it. And god knows he hates doing anything that feeds Rachel's ego. But when he tells his Dad what he did, and why, he knows he'd make the same choice all over again.