It's a figurative use of the idiom be kind enough:
—used to make a request
// Would you be kind enough to show me the way?
In Endgame, it's a kind of sarcastic reference to the past happening in a way that wasn't factual:
- Thanos: "Hey, I'm looking for some volunteers. I need about half of you to die when I snap my fingers. Which of you would be kind enough to do that for me?"
- (many people raise their hands)
- Thanos: "Thank you very much!"
- Rocket (years later): "Those people were kind enough to die."
In reality, permission wasn't asked, nor was it given. It had nothing to do with politeness or anybody deserving to die. That's why it's a sarcastic reference to the event and the dead.