It starts as the power of suggestion and then the thrill becomes addictive. This is a totally understandable phenomenon. The only difference between this and, for instance joy-riding (very prevalent here in Northern Ireland, where young people steal a car from outside your home and take it on a fast ride around the neighbourhood, usually crashing or burning it), is that it is a less sophisticated way of terrorising a neighbourhood. They are both thrill seeking, anti-social activities, where the risk of being caught adds to and heightens the thrill. The added element in the more primitive areas such in Kenya is the suggestion that witchcraft may be the cause, but for those of us with rational minds it seems a clear cut case of deliberate mischief making which becomes habit forming.

In Kenya, witchcraft is still believed by a majority - who are also devout Christians. The same situation that existed in Western societies up to 150 years ago.
The Mau Mau made people attend witchcraft ceremonies, where they had to undergo unpleasant experiences and swear an oath of alliegance. Eventually, the authorities realised they had to employ witchdoctors to perform "cleansing" ceremonies, where those who had undergone the Mau Mau oathing ceremony were subjected to further equally unpleasant experiences. (Merely casting a spell and saying "you're cured" did not have enough "power" to overcome the original oath.)
Modern, city dwellers are less prone to believe in witchcraft and superstition, but rural society is more gullible. An exact parallell to Europe and America, in fact.
It was good to see Lake Victoria in the background.
As an aside, this is the area of Kenya where Barack Obama's father lived.

I didn't see or hear anything that you quoted in the article/video you linked.

Anyway, the same could be asked of any behavior like that. Why do people pretend to be vampires? Why do kids egg houses at night? How do people get enjoyment from manipulating or even torturing another person? Their motives will always be different from myth to myth and even person to person within the same myth.

From the video, it seems like it's a tradition handed down through certain families. They might not even know why it's a tradition, but they'll still do it. Like many people in many traditions, after so many generations, they forget the roots of the tradition. Finding the roots would be key to this myth, but very few probably know. My best guess is that it was myth the was created to scare people. Not just to keep them from coming out at night, but to instill a general sense of fear. Like many many generations ago, the higher ups in the tribe wanted to create something to keep those below them in a state of fear. Much easier to rule over a group of people with them fearing something unknown or supernatural. In today's age, they wouldn't be able to use it as a method of control.. but it still became a tradition.

Sorry to lower the tone but this sounds like great fun. I wonder if there's a local re-enactment society or something I could join...