I potentially saw someone get kidnapped and I did nothing about it.
What did you see happen that made you think someone was getting kidnapped?
I was riding my bike and I saw that injured people were laying on the sidewalk.
When something is occurring that affects public safety such as a potential kidnapping, you should contact the police as soon as possible. Even a small intervention such as making noise and drawing attention could help someone if they are being kidnapped. We all have a part to ensure public safety and being mindful of kidnapping is one of those parts.
I didn't know what to do, so I left
Well you should call the police if you think something bad is happening or people are hurt. It is good to be in a safe place before you make the call but if you see the car or the person try to get a good description of them.