The Armour of Contempt by Dan Abnett
Still haunted by his lengthy mission behind enemy lines on Gereon, Commissar Ibram Gaunt heads back with the Imperial crusade army to liberate the Chaos-held planet. Having made contact with the local elite guerilla forces, Gaunt and the Tanith First-and-Only find that the brutality of the 'liberation' pitches them into opposition with their commanders, who believe victory must be achieved at any price, no matter how cruel. All bets are off as Gaunt and his men are pitched into direct opposition to his commanders!