My elderly mother wants to stop dialysis. What does halacha say about end of life decisions?

This is among the most serious questions a family can face, and it cannot be answered correctly by a stranger on the internet — please bring it to a rav who deals with these cases together with her doctors. What I can offer is the shape of it. Judaism treats life as infinitely precious, and we do not actively end it. But halacha does distinguish between treatments, and the questions of pain, of whether dialysis has become an ongoing affliction, and of your mother's own wishes and capacity all matter enormously.

So gather three voices in one room: a posek experienced in end-of-life shailos, her treating physician, and your mother herself while she can speak for herself. Do not carry this decision alone, and do not let guilt rush it. The fact that you are asking, carefully and in fear of Heaven, tells me you will handle it with the seriousness it deserves.