Chemotherapy and (some) radiation treatments are actually designed to kill patients, it's just that they are designed to kill the cancer first/more. Side effects from most of those treatments is cancer, but since you already have cancer, it's worth the risk.

The hard part about treating cancer, besides from that it's a family/kind of disease and not a single thing, is killing it without killing the patient as well. Everyone has heard about a new cure that kills cancer in a Petri dish, but remember so does a gun. Getting better at targeting the treatment is what has really advanced the field in the past 20 years.

One example is rotating radiation sources which intersect in a 3D point in space. That way the total dose can be high, but the dose received by the healthy parts is low. Only at the focal point the full blast is applied, which is hopefully directly where the cancer is.

Prevention really is the way to go with cancer, by living healthier lives, eating healthier foods and getting vaccines where possible. For example the HPV vaccine is super effective at preventing a specific kind of cancer in women (and for a smaller part men). But getting girls (ages 8-14) to take the vaccine can be hard and the past couple of years have made it harder. Living healthier lives is also easier said than done, with the troubles most people are facing these days it's more of a pie in the sky kinda thing rather than a real option.