Why does the Super Saiyan Transformation expose the Heart to Impurity?
I'm not exactly sure if my answer is what you asking for, but Vegeta tells 19 that when they turn into super saiyan they tend to be more savage. For transforming for the first time you have to be very furious, and probably full of hatred. That might be why Super Saiyan expose the heart to impurity, since to be pure-hearted means to be a person with only good intentions