Where is your evidence? I am looking for hard evidence of the exact dates the drug related crimes were committed, what amount and what type of drugs were involved, the sentences that were in the law at the time the crimes were committed, the date decree 20 was made, the date decree 20 was backdated to, evidence that Buhari was made aware of the principle of non-retroactivity(and that it was a generally and internationally accepted legal principle) and that he ignored this and went ahead with the executions anyway, also the date and place of the executions (and any evidence that may help to further certify them), where the bodies were buried, who was in the firing squad.
This is urgent. If most of the above can be provided then there's a strong possibility Buhari can be prosecuted for murder immediately. There is no statute of limitations on murder and the constitution does not give the Nigerian president immunity from criminal prosecution (only from civil litigation).