So you keep things as simple as possible. "This," you call out, "is a necromancer. The penalty for necromancy, as we all know, is death." You turn to the pyre, regarding the man tied atop it; his mouth is very firmly gagged and his fingers broken to prevent any last-minute spellcasting, and you can barely hear his muffled attempts at screaming from where you are as he thrashes helplessly against the bindings. "The corpses he defiled took a great many of our comrades from us. And they took the life of the Elector Count Abelhelm Van Hal." You're hugely relieved that your voice doesn't crack at that. He can take so much, you reflect, and you can only take his life once. It will have to do. "The penalty for every one of those lives taken, is death."
There are words for this, for ordering an execution; ritual and formality used by judges and witch hunters alike. Van Hal would know the words, but you don't.
So you look at the man, one of the necromancers that raised and directed the horde that had cost you and Stirland so much, and say nothing. You take the torch from the brazier, walk up to him, and hold it to the kindling. It takes barely a second to catch, and you drop the torch and back away as the flames start to climb, banishing the evening chill. You turn and regard the gathered men, who's eyes are universally locked onto the pyre as the necromancer thrashes and screams into his gag. Some few are sickened, but most watch with grim satisfaction. These men are Stirlanders, and have lived in the shadow of Sylvania their entire lives, as their parents have and as their children will. After today, Sylvania will have one less horror to inflict upon the innocent.
You turn back to the pyre, catching glimpses of the necromancer's terrified and agonized expression through the rising flames, and smile.