I am recently reading Jan Potocki's classic "The Manuscript Found in Saragossa", written in the late 1700's or early 1800's. It's a book of stories that deal with the supernatural, the humorous, the ribald, and the scientific. One of the stories tells the tale of the bandit Zoto's father, who was also a bandit. Let me quote from it, and you will see the relationship with Angel Eyes. The narrarator is the younger Zoto.
In Benevento there were two gentlemen, one called Count Montalto, and the other the Marchese Serra. Count Montalto summed my father and promised to give him five hundred sequins if he would assassinate Serra. My father took the commission but asked for time to carry it out because he knew the marchese was very much on his guard.
Two days later the Marchese Serra summoned my father to a lonely spot and said to him, 'Zoto, here is a purse containing five hundred sequins. it is yours. Give me your word of honour that you will murder Montalto.'
My father took the purse and replied, 'My lord, I give you my word of honour that I will kill montalto, but I must tell you that I have given my word to him that I will kill you.'
The marchese laughed and said, 'I sincerely hope that you won't do so.'
My father gravely replied, 'I am very sorry, my lord, I have given my word and I will keep it.'
The marchese leapt back and drew his sword, but my father drew a pistol from his belt and blew the marchese's brains out. He then went to Montalto and told him that his enemy was no more. The count embraced him and gave him the five hundred sequins. My father then admitted to him with some embarassment that before dying, the marchese had given him five hundred sequins to murder the count.
Montalto said that he was delighted to have forestalled his enemy.
'My lord,' replied my father, 'that is neither here nor there. I have given my word.'
And with these words, he struck him down with his dagger...
I have to wonder if Leone was familiar with this story. Probably so, this book is fairly well known, and probably is even better known in Europe.