No one is saying your should explain the motivations of others but in order to put meat on what you are saying, there needs to be reason behind why these individuals would put themselves in such a position. They had the means (I guess) and also the opportunity but what your explanation lacks, is 'motive'. Jeremy had all three means (access the guns and the victims) motive (financial gain) opportunity (they were all sitting ducks at WHF).
The relatives put themselves in that position - partly because they were vultures. Perhaps they also unwittingly put themselves in that position (to the extent that they were victims of their own arrogance and used to getting their own way). They come across as being full of self belief, very judgemental and (like virtully everone else) lacking in knowledge regarding the extent of Sheila's mental health and her circs. It's a bit of a lethal combination for setting off down a blind alley, away from the truth.
We may never know exactly how Jeremy's uncle Bobby managed to 'turn' Peter Simpson.
I think there were four sitting ducks for Sheila. And then there was only one sitting duck left for the relatives.
