My Mother Died Recently From Lung Cancer And During Her

My mother died recently from lung cancer and during her last few days she pulled her oxygen mask off and the nurses said they could not make my mum wear it and that in fact it would be quicker without the oxygen. In fact my mother lived for another 3 days without the oxygen and her death was distressing.....im left wondering if I should have asked to have the oxygen turned back on for her and wonder if this would have made her passing somewhat easier.