How could a "bad" gene-- the mutation that causes the sometimes lethal sickle cell disease -- also be beneficial? On the other hand, if it didn't provide some survival advantage, why had the ...

This DNA has suffered a mutation, either through mis-copying (when its parent cell divided), or through the damaging effects of exposure to radiation or a chemical carcinogen.

In other words, mutations occur randomly with respect to whether their effects are useful. Thus, beneficial DNA changes do not happen more often simply because an organism could benefit from them.