the passuk says when Pinchas killed Zimri and Cosbi – vayakam mitoch ha’edah – he arose from within the congregration.
Apparently this passuk is superfluous – of course if he killed them they he first arose from within the congregation?
The passuk says concerning a beis din who are judging cases of corporal punishment – ve’shaftu ha’edah ve’hitzilu ha’edah – the congregation shall condemn and the congregation shall exonerate.
You see from here that the word edah refers to a beis din.
The halacha is that haboel aramis kanain pogein bo – if someone has relations with a nachri then a zealot may kill them.
Therefore Pinchas had first to arise out of the sanhedrin before he could kill Zimri and Cosbi becuase from a perspective of the beis din he could not kill them, he could only kill them as a private individual and as a zealot.