Just finished The 4400 and, well, did everyone just conveniently forget about Jordan Collier's ability to remove Promicin abilities? Danny Farrell, rather than suiciding, could have had the darned ability yanked out of him by Jordan, and then, oh, look at that - quid pro quo, Shaun would heal Jordan after he'd been exposed to the polonium to get rid of the nanobots.
Very true, but Jordan is also the guy who was all too happy to have Promycin given out like candy— 50% death rate be damned. I'd lay good money on him trying to weaponize Danny instead.
Plus at the time of Danny's death, Jordan was being held captive by the marked with Shawn having no idea where he was or how long it would be till he was free, if it ever happened. Danny was dying slowly and painfully over the next few hours, Shawn just gave him a more dignified exit.