This is a fallacious argument. The reason religious people kill people is because they care deeply about their religion, or more often, they care deeply about the things their religion encourages them to care about. Anybody who cares deeply enough about something is willing to take strong steps to defend it, and in a pinch, they will kill people. Look at people who care deeply about their country. Look at people who care deeply about their sweetheart. You could definitely lower the numbers of deliberate killings if you got rid of romantic love — there would be no more killing due to jealousy. But would it be worth it?
The world would be a safe place if nobody cared passionately about anything. But would it be better?
It’s a serious question, but I’m not sure how to decide it. You could definitely tally up all the bad things that result from caring passionately about things, but if nobody cared strongly about that list, then it would not convince them.