Rape, Pillage and Burn: Happens in various places with various less-disciplined armies, much as it ever does in history (well, probably with less rape, since there are rather more cross-species wars).
The Imperial Military Service, on the other hand, is extremely rigorous about averting this particular trope, to the point of giving any one of their own they catch playing it straight a public field execution right there (see: Kill It With Fire) and being sure to pay the market price for any materials they need to acquire locally. (The reason being, ethics aside, that even back in the day when the Empire was going out conquering its own world, it knew perfectly well that the asset value of its conquest was very dependent on it being annexed in a not-wrecked state and with a population that at the very least hasn’t been provided with many, many reasons why they shouldn’t consider cooperation and assimilation.)
This is a military tradition now, of course, and one which they’re happy to share with anyone on the other side who forgets that War Has Rules (see: Laws and Customs of War). Something that’s particularly important to remember if you hire one of those mercenary companies that promises to sell you the Glorious Military Traditions of Eliéra… because those may just include setting fire to commanders who seem a little too fond of atrocities.