Some people find it quite difficult to admit the fact that blood flagellation is a source of mockery and revulsion. One of the best ways to make a promoter of blood shedding admit this very fact, is by pointing out their double standards with respect to other non-Islamic actions that carry a bad image. You will often find supporters of blood flagellation mocking and ridiculing the actions of other religious people. Criticizing them for giving religion a bad name whilst being blind to the negative image inflicted by blood rituals.
Take the example of the Sufi ritual being demonstrated in the video below. You will find that advocates of blood shedding will be quick to mock these people whilst not accepting the clear fact that the blood shedding rituals are a lot more repulsive and disturbing than what these Sufis are doing.