Probably happened during the Kaled/Thal wars.
I don't have reference to it here but I remember reading (I think it was in the "Genesis of the Daleks" novel by Terrance Dicks) that Davros was born the day that the Kaled/Thal war broke out, roughly 900 years before Genesis was set. He was crippled and slightly mutrated in an explosion when young and set about creating a method of sustaining his life at all costs, designing his own chair/life support system entirely from scratch.
Also, in the original script of "Journey's End", Davros talks about being a medic during the war. While he was looking at dying Kaleds, an bomb hit his bunker, leaving him severely crippled, and permanently scarred by the deaths of his family and friendws. Psychologically, this would have caused a scar which Davros would want to be rid off, and death would have been a treatment, not a cure, to the scar.