Most patients with leukemia die of infection, because the excess white cells do not do what they are supposed to do, fight infection. I don't know about the red cells being abnormal, it is the white cells that grow to large numbers but they are ineffective in killing germs.
Leukemia patients are very prone to bleeding and bruising, and like I say infections. There can be various reasons for death, it is something like patients with AIDS (of course not contracted the same way), but the body just can not fight off infections, very minor infections can become life threatening. Also patients can have strokes because their can be bleeding into the brain among other things.