Genghis Kahn probably thought he was correct in what he did, Hitler also said that he was right in what he did. Esau also thought he was correct to persecute our Patriarch Jacob, and Laban thought he was correct to try and kill Jacob. This does not make any of them correct. And to the Jewish way of thinking, converting a Jew away from Judaism is worse than killing him. Killing someone is bad and wrong, but in the long run it generally harms only the body.
Conversion harms the soul, and is more difficult to repair.
So, when someone writes me, and tries to convince me that I am wrong about Christianity, I will not treat that person with respect and dignity. I will tell him or her exactly how I feel about them. And that is what the Torah tells me to do. There are some good reasons for that. I will tell you one of them.