^ Before that he was a very effective and efficient security officer, who was close to being promoted to full Commander.
But he was a traitor! Even a mediocre yet loyal security officer is better than one that will betray you at the worst possible time!
Not to mention the viruses he put both in the station's computer (found out on time by Odo) and the Defiant which effectively crippled the latter and turned it into a sitting duck!