Paradox of Tolerance

Tolerance is a virtue as everyone can be justified in their actions. 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.