Because he knows Hunter would cashier him and put a stop to his measures if he admits to it, so he tries to get you cashiered instead, because he genuinely believes those measures are necessary to protect his men. And in the grand scheme of things, cashiering one cornet who is unable to get his hands dirty is preferable to ruining the careers of dozens of men who were willing to do the dirty work.
Well, it's not like there was any personal gain in hunting down the partisans - it actually put his own career at risk. And he certainly wasn't doing it because he was bloodthirsty, since at Januskovil he lets the surrendering Antari go free when Katarina wanted them executed. So I think it's safe to assume that he was being truthful in his reasoning/motivation for it.
And if Lefebvre is an ass... what does that make Cazarosta?
Still, not like it was an easy task. The fact that the Grenadier officers knew what they were doing and were competent shows that Lefebvre gave them the right information and assigned them to the right locations, which further demonstrates his capability.
He recognizes that he is a terrible person and has selfless reasons for being one. Compare that to Cazarosta who thinks everything he does is 100% justified, expresses no remorse, and believes anyone who can't stoop to his level is either an incompetent, an obstacle to his own success, or both.