If Jeremy was guilty why would he:
1) Blame the police for killing his family
I have note seen him assert police killed and of his family only wacko supporters of his claim they killed Sheila. But if he ever did claim such his motication would be the same as the motivation of his wacko supporters who assert such- to pretend he didn't do it by blaming someone else. WHy do you ask such silly questions like this?
2) Say to the police he wasn't getting along with his sister
Eithe rbecause he didn't think before he spoke and thus actually spoke the truth or he realized others would reveal the truth to police and didn't want them to catch him in the lie of telling them he got along great with her. Moreover, he figured it would make no difference since he had framed her and expected them to fall for it.
3) Not get an alibi
Because guilty peopel can't have a valid alibi so the best he coudl do in trying to have an alibi was to pretend Nevill phoned him and this was supposed to establish he was home at the time of the murders.
So police would not think he dialed them from WHF he made sure police got to WHF before him.
4) Finish with Julie Mugford a few weeks after he had told her he was responsible for the murders
Julie Mugford could not take it that he was responsible for th emurders and would not marry him and that is why she broke up with him, he didn't dump her. At any rate at that point he considered himself bulletproof and did not think police would be able to prove he did it even if they believed her.
5) Say he was responsible for leaving the gun on the table
Another extremely stupid question. He claimed he left the gun and bullets in the kitchen to frame Sheila. No one would have believed that in the middle of a rage Sheila would go digging through the closet for a gun and bullets and if she had done so then she would have found a gun with the scope and moderator attached and while she was trying to load the gun she coudl have been stopped. The goal was to make it so all she had to do was place a magazine loaded prior into the gun and chamber a round and that's it so there would be no time to stop her as she was trying to load it. Moreover his way if his prints were found on the gun or bullet casings then this would provide an innocent reason for such.
6) Go away on holiday leaving a silencer in the cupboard which was used in the murders
more to be added
The same reaosn he left it there right after the murders- because he had no idea about drawback let alone serology and had no idea they would be able to prove it was used to kill Sheila. He had no idea whether police took it or not originally and if they hadn't he had no reaosn to think they would come back later. But he didn't care anyway becaus ehe had no idea that they could prove it was attached when SHeila was shot and that such would be able to prove she didn't kill herself.