Why was the painting 'The Opening of the Sixth Seal' by Frances Danby damaged?

The Danby painting was exhibited at the British Institution and won an award in 1828. During Danbys lifetime the abolition of slave trade, and slavery came about. In 1807 an act was passed that abolished the trading of slaves. Later in 1833 slavery itself was abolished. These parlimentary acts caused alot of scandal as many did not want slavery to be abolished. If you look at Danby's painting closely you will see a standing figure on the rock ledge in the central forground. His hands are raised and around his wrists you see broken chains. This figure represents a slave. Around him there are figures cowering and dying, some of these wear crowns, and around them are scattered coins and gold. Danby is representing the kings and the wealthy of the world in these figures. In this painting he is making a statement that if the apocolypse comes about it will not matter if you are the richest king in the world, or the poorest slave, all will be judged as equals. This painting was highly popular and it was displayed throughout the country. However during one of these exhibitions, a group of men disillusioned by abolition of slave trade wanted to make statement, and this popular painting offered them a chance to do it. They attacked the painting, and attempted to cut out the figure of the standing slave. They did not want him to be seen as an equal. The painting has since been repaired but at certain angles it is possible to see where the canvas was severed.