I believe that Law still does intend to keep his alliance with the StrawHats but is saying otherwise so that he can fool others into thinking how he wants them too. I also think he's trying to fool DD so that DD doesn't focus on them and worries about just Law.
But with Law having a grudge against Joker, it's possible that this is as far as he's been planning up just to have his revenge on Joker. It wouldn't be surprising but I prefer the alliance with Luffy, I liked their interaction with each other and thought they were an interesting alliance. And I think Luffy and Law are about even right now with Luffy with a slight advantage over Law.