This unfortunate disaster made me wonder whether such an event is possible as a form of assassination.

Perhaps so. In really bad weather, most aircraft use the ILS signals as an approach aid to a runway. The glide slope portion of the radio signal gives vertical guidance; the localizer portion give horizontal guidance. The two taken together create a conical shape, through the center of which an airplane is supposed to fly safely down to the touchdown point on the runway.

Now what if someone meddles with the signals? The glide slope signals are rather fickle. Some misplaced metal near the glide slope transmitter shack can change their angle. An airplane following the erroneous signal could track far below its normal track, and if there happens to be a hill there, bang.

An attentive pilot in a well-equipped plane might notice some signs. His altimeter would indicate lower than usual, and his vertical speed indicator may show a non-constant rate of descent. If he has a radar altimeter, it may warn that he’s too close to the ground. He may have GPWS. On the other hand, a pilot fixated on his ILS displays may not notice those things until too late.

If the assassination team has practiced well ahead of time, they may be able to place a glide-slope-disrupting object in place very quickly – and remove it just as quickly, perhaps by driving a truck right up to the glideslope transmitter shack to a point predetermined to cause the right sort of disruption. This need not even leave a mark behind, so the attack can be tried multiple times without much notice. In thick fog, the local airport security apparatus may not even have to be corrupted, since runways may not be visible from the terminal buildings / tower.