They believe they have the Truth. If you leave the truth, in their view, you will die at Armageddon. I give them credit for not wanting anyone to die. And like the medieval church, they believe that those who leave are a corrupting influence. They imperil the life-prospects of others.
If you do not believe their message, this is nonsense. But it is as they see it. And if you believe their message, this is serious business.