http://www.motorsport.com/f1/news/marko ... ll-engines
This is specifically why Mercedes were correct in deciding not to supply Red Bull with their engines. Red Bull, while being a fantastically technically competent racing team, are toxic in nature largely as a result of the attitudes of Marko, Horner and to a lesser extent Newey.
Renault were a critically important complement to Red Bull's chassis' prowess in the 4 years of domination from 2010 to 2013. The combination of Newey's exhaust blown diffuser related designs together with the Renault engines off-throttle characteristics (developed specifically to suit Newey's goals) and the packaging characteristics of the engine were the reason behind that success.
Yet even during those years of winning, not only did Renault get little thanks and credit for contributing to their success, Horner actually criticised the engine's lack of power continuously. Come the turbo owner, where Renault have actually failed, they got well and truly thrown under the bus. Red Bull publicly slated Renault right from the beginning.
So what's in it for an engine manufacturer to supply Red Bull? If Red Bull wins with a Mercedes engine, then Red Bull will claim how great they are that they can beat the works team. If they don't beat Mercedes, it will be publicly claimed that they are not getting equal engines.
For Red Bull, it's always somebody elses fault. When regulations controlled the exhaust blowing Newey, Horner and Marko were claiming that innovation was being targetted. Contrast this with McLaren who accepted it when the F-duct was banned. When other teams complained about Red Bull's flexi wings, again they were being victimised because of their success.
From that article I linked above where Marko claims it would be more unpleasant for Mercedes if Red Bull beat them with another engine rather than a Mercedes engine. What that acutely highlights is the specific way Marko thinks. It's obvious why he thinks it would be more unpleasant for Mercedes. Because in Marko's mind, it would mean that Red Bull overcame an engine deficit and beat Mercedes because of Red Bull's vastly superior chassis. Marko's wish of unpleasantness on Mercedes isn't one of Red Bull showing Mercedes they could get an engine on par or superior to Mercedes but rather that they can once again show Mercedes how great Red Bull is.
But the reality is that it would not be more unpleasant for Mercedes. Supplying Red Bull would be a lose lose situation for Mercedes. Even getting Ferrari engines is, according to Marko, only a "first step to getting a competitive engine in the future" and he's already thinking about a potential change from a Ferrari engine. For Red Bull, it's not about forming partnerships for the mutual benefit of all stakeholders but rather a supplier is a mere vehicle to assist Red Bull's ambitions and to be used and discarded as they see fit, whilst all the time claiming the kudos. Red Bull is a toxic partner. They are, if you will, like the stereotypical boss from hell who claims all of his employees' success while blaming his employees for failures.