A what if question regarding enemies

So, this just got me wondering and I'm very curious about it. Suppose you know somebody who you thought was your friend but is actually a psychopath/narcissist/machiavellianist (or any other toxic asshole) who utmost priority is to harm you in any way possible just for their own enjoyment. Obviously you'd avoid them and throw them out of your life.

But, what if, there was a terrible road accident or a robber stabbing them and they get injured, and you were the only lying one in the scene watching all this from a distance? Would you choose to save them or let them die just like that?

In my case, I would regret it either way and wish I could have always chose the other option instead.