Immortal cells make for dead people:
There are cancer cells still growing in culture from people who died decades ago. While individual cells die, immortality means those cells keep making new ones to replace them.
Cell death is a mechanism that your body uses to regulate your cells, causing them to behave collectively like a person and not just a giant mass of cells operating each for it's own benefit. Tissues that start becoming cancerous self-destruct. Tissues that shouldn't be in a certain place (like a developmental structure no longer needed) die to allow the collective whole to take on a functional form.
Take away this mechanism, and the cells are a step down the path of behaving selfishly. A muscle cell that has a growth signal makes more muscle cells whether you need them or not - or whether they are even in a functional conformation. Precancerous cells no longer have a signal to stop them from replicating. Without these genes, the body starts to be a mass of individuals who can misbehave without consequences.