What is the meaningful difference between killing someone yourself and letting them die when it's in your power to prevent it? Saying that a certain population

What is the meaningful difference between killing someone yourself and letting them die when it's in your power to prevent it? Saying that a certain population should die because of their criminal nature and high "cost to society" is straight up nazi shit. What's the policy implementation of that opinion just curious? It really doesn't get more clear than this.