It could be done as a murder for hire, or as a terrorist event. It's possible some hackers would do it 'for the lulz' or maybe they just want to test out a hack and accidentally go to far, though these are more unlikely. And there is also extortion, maybe you find everyone that has a vulnerable pacemaker, and demand they pay you or else you stop their heart. Criminals are clever, if they find a way to attack people they can often exploit it.