Led Zeppelin

The Geneva convention wasn't enacted until after the Second World War

The song is called "Kashmir". It's somewhat fitting as a tribute to an Indian mathematician.

Well, why do humans kill humans? The usual reasons, I suppose... greed, racism, misinformation due to propaganda. In the case of air strikes, an additional factor is that even when the official target is a military installation, civilians still die due to the lack of precision. Maybe some pilots did it even on purpose. This leads to revenge bombings and thus to an escalation, in particular if both sides do not really have a problem with killing civilians. The specific case you mention is described as having started this way on wikipedia:

"This was a result of a rapid escalation starting on 24 August 1940, when night bombers aiming for RAF airfields drifted off course and accidentally destroyed several London homes, killing civilians, combined with the UK Prime Minister Winston Churchill's retaliatory bombing of Berlin on the following night. From 7 September 1940, one year into the war, London was systematically bombed by the Luftwaffe for 57 consecutive nights."

https://en.wikipedia.org/wiki/The_Blitz

The BBC adds

"When on the night of 24 August 1940 the German air force - the Luftwaffe - accidentally and against Hitler's orders - dropped some bombs over London, the British prime minister requested a retaliatory raid on Berlin. Hitler responded by going ahead with the Blitz, and the following months and years saw tit-for-tat raids on each country's cities. [...] An investigation revealed that just one in five aircraft was succeeding in dropping its bombs within five miles of its target. Under such circumstances, the bombing offensive could only be effective if it was directed at targets as big as cities. Consequently, in February 1942, Bomber Command was instructed to shift the focus onto the 'morale of the enemy civil population'."

http://www.bbc.co.uk/history/worldwars/wwtwo/area_bombing_01.shtml