A kind reminder

Some hostage takers threaten some hostages with hurting them if they don't cooperate. One of the hostages refuses to cooperate, so the hostage takers kill him.

Hostages: Where is Albert?

Hostage takers: He didn't follow our orders, so we killed him. Let that be a reminder to the rest of you.

1) Is the line in bold perfectly natural in the context?

2) Could I throw in a "kind" before "reminder" or would that only work if the hostage takers had done something to the hostages they are saying this to. Something that is milder (kinder) than killing them as if to say that next time we will kill you?

I hope you understand the second question.