The spy can create a sick eventually ended up with the death. There has been an assassination by a poison for a long time, which is one of the ways to kill a target from a disease. Or, the poison is initially meant to kill a target socially, but it is escalated to a death eventually, though the operative should have thought the death was one of the outcomes from the beginning.
If it is hard to find an evidence of the poison, the target is assumed to decease naturally. However, the tox screen is quite advanced in this modern world, hence it is not easy to hide a true cause of the death, especially in the developed countries. If it is revealed as such, the spy might be a subject of the investigation.
In this way, the poisoning is not an easy option to take by the intelligence organization, though the authoritative government can employ this methodology more often, as they can disguise a tox screen and its investigation itself.
The electromagnetic wave is one of the ways to cause a death from an illness. There are three fatal diseases caused by the wave, the heart attack, brain stroke and pancreatic cancer. If this is exacerbated as planned, the spy can eliminate a target relatively soon. Additionally, the radiowave manipulation can also create visceral diseases, which can be exacerbated to a death as well.
Having said that, not all the targets have suffered from those symptoms. It depends on the individuality, some of those are seriously sickened and some of the others are not even if irradiated the same type of the wave. It is a result of the sensitivity difference for the electromagnetic wave of each organ. It means some of the targets might decease soon after the irradiation and others are more resilient, just to live a life under the unbearable pain.
The electromagnetic wave can kill a subject unintentionally. If you are more sensitive to a certain type of the wave, you can be dead just by its irradiation, even if it is originally meant to manipulate in other ways. The individual sensitiveness cannot be captured beforehand, hence the subject might decease from the heart attack or brain stroke suddenly.
Having said that, the intelligence organization originally knows the possibility of these deaths. They have conducted numerous operations and fully realized some part of our population are easily killed by the wave.
In the modern legal system, this is treated as an intentional murder, as the operative knows its possibility beforehand. However, the electromagnetic wave capability is not officially recognized, hence the intelligence organization takes no responsibility for its outcome, which makes us quite in danger.