What sort of ulterior motives could Kaworu have when he told Shinji that everything was his fault and that everyone "hates him" for it?
Kaworu is described as Seele's boy by Fuyutsuki. As far as we know, his intentions and Seele's are more or less the same. When Kaworu lured Shinji into Terminal Dogma to retrieve the spears, Seele was confident that their plan was being fulfilled and they consented to being shut down by Gendo in order to join in Human Instrumentality.
By placing the blame on Shinji, Kaworu ensures that Shinji will be more desperate and willing to "fix things" by retrieving the spears. When you start to look at it like that, everything
about Kaworu becomes suspect, including how unconditionally kind to Shinji he is. He's been manipulating Shinji the entire time in order to serve Seele's end. However, I also think that Kaworu genuinely believes Seele's plan is the best thing for Shinji, the key to his happiness, which is Kaworu's given motivation from the beginning.