That's true. But how did he figure Sonny would react to the murder of his father? Surely he didn't think he would just blow it off as business. Also,he knew that Sonny would not necessarily succeed his father as Don. So, he must have figured that Clemenza or Tessio might go for the deal if Vito and Sonny were out of the way.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
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