Why did he go to the Spanish War?

In George Orwell’s famous essay "Inside the Whale" there are some sentences which Orwell seems to have written, apart from the main theme, namely, in parenthesis, but which I have been deeply impressed by : "It so happens that I have seen the bodies of numbers of murdered men--I don’t mean killed in battle, I mean murdered".
Incidentally, also in "Shooting an Elephant", similar sentences are inserted in the same way: "Never tell me, by the way, that the dead look peaceful. Most of the corpses I have seen looked devilish."
What do these sentences mean? In 1922, Orwell went to Burma as assistant district superintendent in the Indian Imperial Police. He served in a number of country stations and at first appeared to be a model imperial servant. Yet as he realized how much against their will the Burmese were ruled by the British, he felt increasingly ashamed of his role as a colonial police officer.
As he writes in "Shooting an Elephant", "The wretched prisoners huddling in