He didn't just want to make Nunnally happen, he wanted a "fair" world or whatever. He succeeded in that.
He could be thinking that he made Nunnally happy with the "funnel of hatred" plan. He geassed her to solidify her hate for him, and then died thinking she would be happy as a result. Too bad she can read minds by touching hands.
Alternatively, he was just being pushed into the plan by Suzaku, who doesn't care about Nunnally nearly as much if at all anymore. He thought up the plan when he thought Nunnally was dead, and when he found she was alive, Suzaku made him proceed ("cut away his weaknesses").
But yeah, Suzaku would make a crappy Zero. Presumably there's not too much Zeroing to do anymore, other than just being a symbol.
****, as far as I can tell, the name "Zero" is just meant to imply that he's Britannian. So yeah, it's lost all meaning...