Dame Janet Smith saw Shipman's attempt to manipulate the will as a decisive act.
//She said he might have deliberately bungled the forging of the £360,000 will that eventually trapped him because he felt "an overwhelming need to stop killing."
She went on: "Either his plan would succeed and he would run away with the money, or he would be caught. Either way, the killing would be stopped."//
Full attempt at trying to fathom his motives here.Old but still relevant.
http://www.telegraph.co.uk/news/uknews/1401977/The-question-remains-why-did-he-kill.html
It won't be the same for all but I think power is strongly to the fore in many of these cases.