So to date, has Doc U or Greta been shown harming anyone in a way that doesn't specifically further their objectives?
I don't think Universe was going to let her die from the beginning. Not that he woudlnt use her as a reactor after her death, or that he wasn't hoping she would steal the C-K reactor for him, but I think he's simply not that much of a bastard in the first place. And anyway a favor can be a huge bargaining chip.
Even Evil has standards.
Either that or it's
More than mind control