SEELE; the throne of souls
Kaworu's first appearance in the manga is at the end of Volume 7, after Shinji defeats Zeruel, consuming the Angel's S2 Engine and throwing a wrench into SEELE's plans. The old men gather and plot how to reclaim control of their plans toward Human Instrumentality, which is to unveil their trump card: Kaworu himself. They address him by his Angel name, Tabris, and his introduction to readers is from inside of a test tube. In the manga we can clearly see how much restriction is held over Kaworu—naked, and much in the same form and display as Rei's dummy plug system core.
[...] That man is at the end of his usefulness. But Ikari... he is no fool. He may have already surmised our intentions. It is time. The keystone of our plans. Let our trump be seen. Let him be our striking hand. SEELE, Stage 48
There is no doubt that Kaworu relished his temporary freedom when he was sent out. Where the anime was vague, the manga showed just how much Kaworu knew he was a pawn in SEELE's plans. When SEELE speaks with Kaworu for the last time before sending him to Central Dogma, they remind him that their hope is now with him. To this, Kaworu replies, "I know... For that is the reason I have existed from the start."
I think it's safe to say that Kaworu held no loyalty toward SEELE. He knew they were using him as a tool, and he was aware he was disposable—he voiced this fact occasionally. When SEELE saw Kaworu for the last time, before they sent him off to perform his destiny, they asked, "You have a sour face. Is something the matter?" To which Kaworu replied sardonicly, "No, nothing. And my face doesn't matter, does it?" He knew his body didn't matter; it was provided to him by SEELE, and there was no value in it to SEELE other than to initiate Third Impact. More on his origins can be found here.
There is a man who seeks to gain power equal to god's. There is a man who seeks to open Pandora's Box before and then to close it again before the hope contained therein can appear. SEELE, Stage 71
One solid fact from the story of Evangelion is that the point of the Eva Units was not to defeat the Angels and save mankind. That was just an additional duty they were able to perform. The true purpose of the Eva series (by series I don't mean the anime, but the series of units) was to bring mankind into Third Impact. This is SEELE's ultimate goal—perhaps its only goal. Since SEELE regarded Third Impact as the only way for man to evolve and purify itself from its past sins. This was mankind's only hope.
Hope? Are you saying that is the hope of all Lilim, and not just yours? Nagisa Kaworu, Stage 71
To this hope, Kaworu only felt skepticism and apathy. It's with little surprise that he would not care for Third Impact—it had no benefits for him. Even though Kaworu did not care, his skepticism was for SEELE's arrogance that this was what all Lilim—humans, descendants of Lilith—would want.
When Kaworu betrayed SEELE's plan in the end, it was a blow to the committee, though they recovered with little time. They settled the matter by postulating that perhaps the last Angel, Tabris, was not meant to succeed. Human Instrumentality must be instituted by their own hand.