Sometimes these mutations are harmless. Sometimes they simply kill the cell which usually isn't too bad, if not too many cells get killed. But sometimes the cell mutates into something that reproduces itself quickly. Mutated cells multiply and take over nearby cells or break off and travel through the bloodstream and lodge elsewhere in the body, or stay in the blood and destroy the blood itself. If a reproductive cell is mutated, genetic mutations are possible as well, and offspring suffer the consequences.
Mutation is a chancy business.
Mutation is not necessarily a bad thing, but it usually is a bad thing. In fact, the vast majority of times, it's a bad thing. The human being is an awesomely sophisticated entity. It is one of the longest living animals, certainly the most intelligent. It thinks it's the most compassionate.