That just means Ruby is a bit of a shitty person. She heard from the Doctor about the nukes, she saw the interview on TV, with him talking about nukes, and allowed a woman to be sexually assaulted before taking action, by not taking action beforehand.
I just think she should have acted as soon as she got the chance to be 73 yards from him. I'm totally sure she had some chances beforehand. Just doesn't smell right to me.
What if she only gets one chance? What if taking out the wrong person means she also fails? What if things weren't 100% set in stone for the PM until the day he's about to receive the nuke codes?
If I was in Ruby's position, trying to figure out what the hell is going, I would also want to absolutely make sure it was the right choice. It's either that or saying fuck it, YOLO (and if she guessed wrong, that's literally it). I really don't see any issue with it, it's by far the logical choice.