I waited until both Belethor and Sigurd were indoors. I used Shadowcloak to go invisible in the corner before firing a frenzy bolt into the room they were both in.
The carnage began. These two workmates, and presumably friends, started swinging wildly at each other, while I sat and watched, enjoying every second. Before too long, Belethor got the upper hand and killed Sigurd. This wasn't enough however, so I reanimated Sigurds lifeless corpse, turning him against Belethor again. This time Belethor ran out of strength, dying at the hands of my new thrall. I remained undetected throughout the whole thing.
The contract is done. Belethors broken corpse lies on the floor of his shop, and his assistant Sigurd is now in eternal service to me as an undead thrall.