In 1885, Khartoum fell to the Mahdist Sudanese rebels. These were people who were rebelling against the Egyptians and British for nationalistic and religious reasons. The city was besieged and eventually fell. Its defenders all died, including General Charles Gordon who was the leader of the defenders and the Governor-General of Sudan.
The event that is most likely to be seen as revenge for Khartoum is the Battle of Omdurman. This battle was fought in 1898. The British commander in that battle was Lord Kitchener. It is said that 10,000 Sudanese were killed in this battle. These were people who had very little in the way of modern weapons and were going up against British troops armed with things like rapid-fire cannon, machine guns, and gunboats.