I think that they are so convinced of their own beliefs, that they leave no room for the validity of any one else's beliefs. I don't know if they are so much "Mean" as they are of a "closed mind"! As such, they can say some very hurtful, unforgiving things!
I agree, totally, with Oluwa about punishment. If you look at it in those terms, then their beliefs do appear to have limited plausibility!
We live in an imperfect world, our environment is degrading, and our well being is on a precipice (at best). As such, we fall victim to nature, the environment and the fallibility of our fragile bodies. However, it is our soul that is precious and that is loved! If our bodies are weakened or diseases, this is not a punishment from God...at least not the God that I love and who loves me!
Peace and Blessings
Look For The Good and Praise It!