god, that's horrible!! and to think that there are people out there who can willingly kill precious, innocent lives and not be remorseful. I wonder why she even bothered to adopt them in the first place?
Please don't beat yourself up too much, you didn't know. Had you know, you would never have let them go to her. It's not your fault. You're doing such good by helping these kittens grow and thrive when their mothers should have done the job. You're helping other cats have the chance at a life they would never have if caring people didn't step in.
Please keep up the good work, and again, please don't blame yourself. There is a special place for people who bring harm to animals, and she'll be going there when it's her time.