Demandred has his own plans and watching over Mat, guiding him (or his army more likely) could fit into them better. We haven't been told much of what Demandred has been up to and have very few clues towards his intentions other than becoming Nae'blis. It could very much be that he would rather try to turn Mat, a powerful ta'veren and friend of the Dragon rather than kill him off. Think of what that could do for the Shadow come Tarmon Gaidon if one of the three, the one with arguably the best battle knowledge and the luckiest son of a bitch alive were to be on their side. It could very well turn the tide of the battle and gain the Dark One's favor for Demandred.