Kill It With Fire: Played straight in both senses. The Empire is certainly happy to use flamethrowers and other Fire Breathing Weapons, fire bombs, fire sheets, and other kinds of fire in war, not because it’s tremendously efficient (it’s not) or because things are Immune To Bullets (they aren’t) – but rather, just because while it’s possible to convince a lot of species to charge down the artillery, or even to charge down the machine guns, a good 95% of everybody is scared shitless of fire and just won’t charge down a wall of it. Fire is terrifying, and the more you scare them, the less you actually have to kill them. Of course, it works well enough for that, too.
(Actually, there is one thing they are really good at killing, which is another reason they’re kept on the active list – nanoweapons. Remember, nanites in general are constrained by their ability to dump heat. How do you add a lot of heat to a nanocloud quickly? Yep, flamethrowers. Big ones.)
They are not impressed by the “it’s inhumane” argument. It’s war. Of course it’s not bloody humane. And the guy over on the other battlefield trying to stay conscious through the agony so he can keep holding his guts inside him with his fingers after being disemboweled by slugthrower fire is not having a significantly better a day than the one who got flamed, m’kay, because that’s not really possible.
In the other sense, yes, symbolically, fire cleanses and transforms. It used to be used by some religions this way – it’s how the Somárans executed the seeress Merriéle back in the day, not that that went terribly well, and it’s also in this form, the fires of purification, still used by the judicial system for particularly abominable criminals, the ones who put a great deal of effort into demonstrating that they were corrupt right down to the soul – the serial killers and torturers and rapists and other absolute dross of society.
Of course, these days said fires are no longer a conventional pyre, good at making a point as those might have been, and is actually quite fast and humane. They involve a sealed chamber containing a fusion torch, into which the criminal is placed – and then, shortly thereafter, it again contains only the fusion torch.