Cancer cells are immortal only in the sense that they escape the built in cellular programming designed to make cells die when they incur irreparable damage. That and they usually express telomerase, which extends the telomere's (chromosomal endcaps) which allows for unlimited mitosis (escapes the Hayflick limit).

All cells can die. It isn't even slightly difficult to kill cancer cells. What is difficult about cancer is killing ONLY cancer cells, and/or ALL cancer cells.