Brochas Daf Zayin amud aleph

In the neighbourhood of R. Joshua b. Levi there was a Sadducee  who used to annoy him very much with [his interpretations of] texts. One day the Rabbi took a cock, placed it between the legs of his bed and watched it. He thought: When this moment arrives I shall curse him. When the moment arrived he was dozing [On waking up] he said: We learn from this that it is not proper to act in such a way. It is written: And His tender mercies are over all His works. And it is further written: Neither is it good for the righteous to punish.

The verses suggest R. Yoshua's plan was wrong, but why? The Sadducee had debased God. Why wasn't he subject to the death penalty?

The Gra explains that it was not the punishment which was problematic, but rather the involvement of R. Yehoshua who had himself suffered at the hands of the Sadducee . Unless a judgement can be rendered without a hint of an ulterior motive --clearly  impossible if the judge is also the victim -- the judge will himself be guilty of Shfichus Damim and hence subject to the death penalty. That R. Yehoshua ben Levi nodded off at the crucial moment was a chessed from the Ribbono shel Olam.