You're offered 400 million dollars by one of the most evil serial killers in history.
This person has committed so many horrific crimes that reading the details would make most people sick to their stomach. He has never harmed you or anyone you care about, and he has absolutely no intention of doing so.
You are the only person who possesses a piece of evidence that would guarantee his conviction and put him in prison for the rest of his life. Without your evidence, he will be found not guilty and walk free.
Before the trial, he secretly offers you 400 million dollars to stay silent until the verdict is reached. If you accept, he will pay you every cent, guaranteed. He will never target you, your family, your friends, or anyone connected to you, either directly or indirectly. There is no trick, no betrayal, and no hidden catch.
If he is convicted because of your evidence, you receive nothing. He also has powerful criminal connections, and it is widely believed that many of his partners are still out there and have never been caught. While there is no guarantee they would come after you, turning him in could put you on their radar.
If you stay silent, he walks free with no legal consequences.
What are you doing?
Edit: 100 percent he will kill again just to be clear. If you let him go.