I witnessed some crash their bike and didn't stop.
Do you think the person was injured?
Probably, because it was a pretty bad crash.
You should have stopped to check if the person that crashed their bike was okay. If you think someone might be hurt, the right thing to do is  stop and check on them.
Are you sure I should have did that?
Yes of course, they could have been seriously hurt and you could have called 911. Next time you really should make sure they are okay.