Primordials don't work like that. They are not Touhou characters who can use twists of wording to apply their themes to anything and everything - and in fact, they're not that bound to a single theme. The Ebon Dragon is more than just "treachery backstabbing dickery", not everything he does is based around those things, and he is under no compulsion to have everything he does ever cause failure. His Excellency has more to do with corruption, lies, enlightened self-interest (making him relatively "trustworthy" as long as you remember who you're dealing with) and antagonism than betrayal, though betrayal is certainly something that falls within his themes.
The conception of Primordials as gigantic 8-balls with only one setting is one of the annoying things that's been slowly happening to the franchise. They're a hell of a lot more complex than that, and act in more complex and many-layered ways than the highest of gods.
The Ebon Dragon, too - people seem to be misunderstanding his themes. He has more to do with lies and corruption and escape than betrayal, though it certainly falls under his themes. And his "failure" thing... basically, he defines "success" not as being the highest of the heap, but as everyone else being lower. Which in turn means that he doesn't care where he ends up, even if he loses, as long as everyone else loses more. But by the same principle, he doesn't care if everyone wins, as long as he wins the most. He is not compelled to make everything lose. He acts in enlightened self-interest. He is even trustworthy to an extent, as long as you remember who you're dealing with - as long as he stands to gain more from a bargain or alliance than you do, even if you still stand to gain a lot, he'll see no reason not to keep to it.
He is not the gigantic principle of cosmic failure and dickery. That's the flanderised version of him.