Paradox of Tolerance

Tolerance is a virtue as everyone can be justified in their actions. For instance, is a criminal simply someone who is mentally ill? i.e. they were born with an enlarged part of the brain which gave them an overbearing propensity to break the law?

The paradox of tolerance states that if a society is tolerant without limit, its ability to be tolerant is eventually seized or destroyed by the intolerant. It is paradoxical in the idea that in order to maintain a tolerant society, the society must retain the right to be intolerant of intolerance. A thought experiment, to show your extent of tolerance is looking at where you draw a line, what do you think of is a crime? Why could the propensity of this crime just be thought to be a mental illness which needs treatment.