VX is an extraordinarily potent chemical weapon. About 0.01g – less than a drop – on the skin can kill. The chemical goes through the skin and disrupts the nerve system.
It is an oil-like substance; it would normally not mix well with water, which raises questions about how it was applied on Mr Kim without killing those who carried out the mission. This is the first time VX has been used in such a way, so there are plenty of imponderables.
The CCTV footage and police statements do not lay out the full sequence of events. Two women appear to assault him, at least one with a cloth wiping his face.