שמואל א כה:לא פירוש: רש״י

That this not be…an obstacle. As a stumbling block, as in, 'and the stumbling [לְפִיק] of the knees';²¹Nachum 2:11. for had you done this, you would not be able to open your mouth to reprove anyone in future generations, concerning bloodshed.

And that my master has avenged himself. That you avenge yourself.