She will ask you to help her with her plan after the race in Santo Domingo. You should put any moral qualms aside because Claire is willing to give you the Beast if you help her out. All you need to do is help her ice Sampson, and you've probably killed a hundred people in Night City by this point for less.

Should I let Claire kill Sampson?

You can stay Claire's hand temporarily to hear his side of the story, but no matter what you choose, Clair we'll shoot him. Even if you tell her to let Sampson go, his fate is sealed. After the deed is done, accept Claire's offer to give you a ride.

How do I convince Claire not to kill Sampson?

You can tell Claire to "Let him go," or you can tell her to "Do what you have to do." If you chose "My priority is winning" after the Santo Domingo race when speaking to Claire at the dam, you'll be given an additional dialogue option and if you follow that line of dialogue, you can convince Claire not to kill Sampson.