You would think he would already have done that before Data got hold of the disruptor.
As I said, he was cocky, which was fortunate.
But in spite of that, I don't think Data's preservation of his freedom was his main concern when deciding to fire. It was the continued threat to others that Fajo represented.
Of course, however, the next question is, why did he lie about it afterwards. That would seem to imply that even though he thought it was necessary, it still violated parameters of his ethical subroutines. And the fact that he could lie about it afterwards would really only compound the unethical behavior, wouldn't it.
Fajo was probably right that that sort of killing violated certain parameters in Data's subroutines, but just wrong that Data would lack the means to be able to overcome the limitations in such circumstances.