Short answers, pulled from the story.
The Jallianwala Bagh massacre took place on the 13th of April 1919 in Amritsar, Punjab, British India. It occurred on the day of the annual Baisakhi festival, when a crowd had gathered to celebrate and to protest the Rowlatt Act.
Estimates of those killed in the Jallianwala Bagh massacre range from 379 to more than 1,500. The Hunter Commission confirmed the deaths of 337 men, 41 boys, and a six-week-old baby, while the Indian National Congress put the death toll at more than 1,000. Over 1,200 others were injured, of whom 192 sustained serious wounds.
Brigadier-General Reginald Dyer ordered his troops to fire at the crowd. He later told the Hunter Commission that his purpose was not to disperse the meeting but "to punish the Indians for disobedience." He was relieved of his command on the 23rd of March 1920 and died in 1927.
The Hunter Commission, formed on the 14th of October 1919 and named after its chairman William, Lord Hunter, released a six-volume report on the 8th of March 1920. It unanimously found that Dyer had committed "a grave error" by continuing to fire, had exceeded his authority, and that there had been no conspiracy to overthrow British rule in Punjab that would have justified his actions.
Udham Singh, who had been present at Jallianwala Bagh on the 13th of April 1919 and was wounded there, shot and killed Michael O'Dwyer at Caxton Hall in London on the 13th of March 1940. O'Dwyer had been the lieutenant-governor of Punjab who approved Dyer's action. At his trial, Singh said he had spent 21 years trying to wreak vengeance against the man he called "the real culprit." Singh was hanged on the 31st of July 1940.
Britain has never formally apologised for the Jallianwala Bagh massacre. Winston Churchill condemned it in the House of Commons on the 8th of July 1920. David Cameron, visiting the memorial in February 2013, called it "a deeply shameful event" but did not apologise. Theresa May described the 1919 shooting as a "shameful scar" at the centenary ceremony on the 12th of April 2019. In 2019, Britain expressed deep "regret" but stopped short of a formal apology.