They take the Geass because they desire power, and it is a way to give it to them. Yet, an eventuality is that they take over the position of the person they contracted with, and gain immortality.
Seems cool at first, but then an eternity passing while you cannot die nor age becomes a living hell, where you're overwhelmed by all the change yet unable to be different yourself.
Passing on the immortality, the person they contract with has to be the one to kill them. If the person is too weak, without enough resolve, they themselves will be killed. That's what ended up happening to people like Mao; he couldn't bring himself to kill her, so she took pity and left him alone.