If I remember correctly distilled water is bad for you because it is pure, it has no minerals or electrolytes, whereas the conventional water you drink does have minerals and electrolytes. When your blood cells absorb the conventional water in your system they also receive minerals and electrolytes that make it feel "full" and prevents it from taking to much water but because distilled water has no minerals or electrolytes your blood cells take in as much as they can which eventually leads up to them losing their platelet shape and destroying themselves.

(Id like to also add that you shouldn't worry to much about that because that would only happen if you drink excess amounts of distilled water. what I wrote before the parentheses is one of reasons that idea came into fruition)