The day she came was one of a blessing and a curse. Who would've imagined that the new "special" student shall be indebted to the club? Certainly not him; no matter how devilish he is in nature.
Kyouya then secretly became the "dark horse" in the quest for Haruhi's heart. Without informing the others of his plan, he contacted Haruhi's family directly.
He was surprised to see that the boyish girl had been a stunning, gorgeous beauty back in middle school. But, he couldn't expect less from his tastes. By then, Haruhi had already captured his heart whole.
So, unknowingly, he became truly gentle with Haruhi. The only female he was going to be gentle to. Well, maybe except his sister.
Kyouya thought that he was doing well in his subtle advances without anyone knowing. Little did he know that the two seniors of the club had noticed it already. So it came as a surprise to him when Huni called him for his actions. Needless to say, he was forced to say half-lies to save his dignity. But he knew that the two didn't believe a word that he had said. Not even a bit.
But Kyouya knew that he couldn't continue his advances in Haruhi. She was, after all, quite suspicious of him already. And also, he couldn't bear to give grief to his bestfriend; even if it contradicts his title.
For him, Haruhi was the only girl to have made her way into his heart, with little to no effort at that. She was unlike any other woman in his society, or in special cases, her society. But that's all the more reason for him not to do anything to her. Though he knew that Haruhi would still be his friend even if his feelings were made public, he can't bear to lose the already little interaction he has with her. It was already few as it is.
Then there was Tamaki. Tamaki was and is his bestfriend. He can never betray his friend. He was already using him as it were. It would be unfair if he did anything else.
In the end, he came to a very bitter conclusion. He can never have Haruhi. If he was ever going to be selfless, it would be this one time only. He'll let the two most important persons in his life be happy with each other.
If it's for them, he'd do anything. But making them sad, he just can't do it. He'd kill anyone who's stupid enough to try to.