Who did Udham Singh assassinate?

Udham Singh assassinated Sir Michael O'Dwyer.

On March 13, 1940, Udham Singh shot and killed Michael O'Dwyer at Caxton Hall in London. He did this to avenge the 1919 Jallianwala Bagh massacre. It's important to note that O'Dwyer was the Lieutenant Governor of Punjab at the time of the massacre and had endorsed the actions of General Reginald Dyer, who had ordered the troops to fire on the unarmed crowd. Udham Singh held O'Dwyer responsible for the tragedy.