What kind of immortality are we talking, here? IS it just an extended lifespan, or could I survive instances that others cannot? Could I die from a gunshot wound? Blood loss? Lots of serious pain? Decapitation?
Even going Kira on people would be difficult if death is possible via injury, especially since you need to kill personally. What use is another year of like when my victim's criminal buddies are after me? Telekinesis would help to a degree, but that's different from omniscience. I could still be caught off guard or unawares, Everyone has to sleep sometime, and I have to know someone is there to stop their attacks with my mind.
In the end, even if you want to be noble about it and only kill criminals, it's too risky, and certainly not worth the cost.
Now if I was completely untouchable.. I would probably do it. I'm not arrogant enough to think that it wouldn't drive me insane or that I wouldn't eventually start murdering small time criminals for lack of other options under the misguided premise that "all crime is equal..." I just know my personality well, and have a feeling that I'd prolly fall into a serious tragedy if faced with this choice.
Now how about something like... "Every time you eat a jelly bean you gain one year of life?" I might do that for a while.