Oh, she got too attached to the source of the Guardians' powers when she was leading the team years ago. Oracle decided she should give it up and hoped that it would wean her off of it.

Long story short, she kills the next holder - her best friend - gets imprisoned, then freed to go blow stuff up.

Oh, and ends up siring a child that ends up being one of our best friends and one of my friend's boyfriends, but that's another story...