I used to have a mystery snail in each of my tanks, but then my female betta Ice kept Beating up her poor mystery, It was bad. Even worse she was actually eating the parts she ripped off, her snail never came out anymore. But he was NOT dead. I thought maybe she would get used to him, and stop eating him, But apparently not, So I took the poor little guy out. Now my male Marius, Has two mysteries in his tank. He doesn't bother them at all. Infact, the snails will be by each other, and marius will go up and rest on them! lol.
I love all my snails, Infact, I have two tanks with just snails in them, and one that I gave to my dad. He likes em too lol.
So, I guess it just depends on the betta.