There's always the possiblility to consider Chaos to be complex beyond mortal ken, who think it is completely disorderly, random, but in truth its rules are just much more elaborate than those of order.
So demons' deeds are only seemingly random. Others consider them random, they themselves - only agents of Chaos - might even consider them random, but they ultimately follow rules or a Great Plan.
So that Demon appeared on the Prime, killed a bunch of people, spread some terror, but was slain by paladins before it could do really great harm, like that devil who managed to turn a whole church evil by making them persecute, torture and kill everyone who doesn't agree to their views? That devil accomplished a lot more, right?
Wrong! The demon might just have killed a couple of people and frightened another few people, but one of the people killed would have sired the grandfather of one of the greatest heroes of mankind (who would have abolished poverty and brought a thousand years of piece to the nations of the central kingdoms), were he not killed by a "random" demon attack. Oh, and that one guy who had to be commited because of his panic attacks? A decade later, he was released as the clerics of the sanitarium considered him sane enough to be sent out into the world again, but he lost all faith in good when he saw his wife and children being brutally killed by a nether horror. This man, a keen businessman, was fated to open a very successful business 15 years after the moment of attack, and that he will still do. But instead of the importer of spices he would have become, he started to trade in slaves, quickly gained enough political influence to make slavery legal in his country, and expanded his slave trade greatly, bringing misery to thousands.
Or maybe, one of the victims was the distant ancestor of the Great Hero who would have turned back the great Demon Incursion 10.000 years hence. That hero, and indeed all the 500 men separating him from his ancestor, will never be born, and in 10.000 years, the Abyss will annex the Material Plane. No more than 2000 years later, they will have crushed all the other planes from there.