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Many famous people in history died of allegedly natural causes and were suspected of being poisoned.
Thus there are many people in history who might have been assassinated by poison if poisoning is counted as an assassination method. In these cases there were often persons suspected of ordering the poisoning but the actual poisoners were usually unknown.
We may suspect that some of the alleged victims were actually poisoned and in some of those cases the poisoners worked for payment and were more or less poisoners for hire.
Guilia Tofana (executed 1659) was an Italian professional poisoner who allegedly sold poison to hundreds of women to murder their husbands.
Locusta (executed AD 69) was a famous alleged professional poisoner in ancient Rome.
In the Affair of the Poisons in 1677-1682 many people were arrested and tried on charges of witchcraft and murder by poison. Those convicted and executed included alleged professional poisoners such as Marie Bosse and La Voisin.
Some historic people died in possible accidents such as King William II of England, shot while hunting in 1100. Since nobody knows if his death was accidental or murder, nobody knows if someone paid someone else to kill him.
And other people were murdered and the identity of the murder is not known, for example King Wenceslaus III of Bohemia and Poland in 4 August 1306. If the identity of the killer is not known, nobody can know if he was paid by someone else to kill the victim.