Will a taser kill someone who has an implanted defibrillator?

Maybe not: A taser might damage implanted devices, although some studies done on pigs found the 5-seconds of high voltage did not damage pacemakers or implanted defibrillators. Those devices likely have safety features to block high voltage electrical currents. Not sure if a defribillator could reset the heart if a taser created an abnormal rhythm, but the studies suggest the defribrillator would still work.

No: There are reported deaths due to taser use, presumably because the taser provoked a lethal cardiac arrhythmia (ventricular fibrillation). An implanted defibrillator would treat that - so, if anything, a taser would be "safer" in someone with an icd. I'm not aware that it's been tested and wouldn't count on it, though. Depending on the vector of the shock, it could theoretically deactivate the icd.