everybody seems to forget the first thing that Pincus did prior to getting his spear and killing Zimri. First he checked with Da’as Torah. Rashi said thar Pincus went to Moshe and reminded him of he halacha and Moshe told him that being that the halacha was learned (remembered) through Pincus, Pincus should be the one to carry it out.
The point is that Pincus KNEW the halacha yet he did not act on his own without first checking with Da’as Torah. That is a valuable lesson in zealotry – even if you KNOW that you are right, stop and confirm first. By Pincus, time was of the essence and he still stopped and asked.