He didn't need to be a genius to realise his mother would inherit from her own mother. But if this is being used as an argument in defence, you also have to consider that it is also an argument in support of guilt.
This is why I don't like how most courts apply the slayer rule and what rules have been put forth by legislatures.
The language of most laws only permits preventing receiving property owned by the victims not future property that would have passed to the victims.
So even despite the conviction Jeremy still could have inherited from Granny if she had not changed her will.
This is particulary stupid when you consider that some jurisdictions do not allow the children/grandchildren of murderers to inherit either. In 2011 UK law was amended to permit children of killers to inherit the estates of victims. In the US though most state laws do not specify and leave it up to a court to decide and at leat 3 states bar chirldren of killers from receiving on public policy grounds.
Basically the argument goes that some will kill in order to provide for their children. I personally think that is a very stupid rule and that uless their kids were involved they should not be barred so like the UK law.
In contrast, it makes sense to say a killer should be considered predeceased when a victim he killed is passed over and thus an inheritance goes to the killer instead. Some say this is unfair because the victim could have died from some other cause anyway but I say who cares let them be unable to inherit period when their victim is in the inheritance chain as a deterrence to committing murder.