HELP!

So this isn't baby related but, I need some advice! I'm feeling so guilty and upset with myself for not saying something sooner but, I'm not one for confrontation. Advice??

BACKSTORY:

I work in the childcare field and I saw another coworker do something that was unacceptable in my eyes but, I didn't want to step on her toes, and ignored it for the rest of the day. I then thought about it last night, and it kept me up most of the night. I went into work this morning and I told my superior. She was completely disturbed by what the other coworker did, and rightfully so. But, she was also, upset with myself and others who saw it happen but not say anything.

I've never thrown someone "under the bus", and Im feeling so guilty about it! Was i in the wrong? Should I tell that coworker about it before she gets "talked" to?