King Kennith of Scotland was killed in 995 and his throne taken by King Constantine

The reign of Kenneth II lasted from 971-995. In 994, he tried to march into Northumbria on some conquest venture, and failed. He returned home to find an uprising in progress. Here is a relevant quote from the book "Monarchs of Scotland":

"There is a story that Kenneth met a startling death. For a long time the Scottish kings had found the Mearn a difficult area to control and in attempting to force his authority there Kenneth had slain the son of Finella, the wife of the mormaer of the Mearns. The embittered woman planned a sinister revenge. In her castle she had a copper tower built, bedecked inside with rich and elaborate tapestries. Several loaded crossbows were secured behind these hangings. She then placed a statue of the king holding a golden apple in the center of the room, at which point the bows were aimed..."

Constantine III (also of the Avalon ep) followed as king. His reign was "as unsuccessful as it was brief...some say that he was killed by an illegitimate son of Malcolm I...Constantine's death marks a low point in the early story of the Scottish monarchy..."

For more info than is presented here check out a book called "Monarchs of Scotland" by Stewart Ross, it has info about the MacAlpin line as well as a family tree. There was no Princess Katherine or Prince Malcolm of course, but they might be cousins somewhere in the line.