Am an I asshole?

[OddBallFreak]

If they were going to fire him instead of him just getting a demotion, I could see it being a real dick move to not give him a heads up. Were you explicitly told (or are you fairly confident) that if you were to say something to him, it puts your chance for advancement at risk? If so, the guy should be understanding enough to see the spot you were put in and not hold a grudge for it. If he isn't able to see things from your perspective and decides not to like you for not telling him, then he's the one being an asshole and good for you being in a better spot because of it.

As far as dealing with the people that are going to work under you and how they might be upset, It's shitty but it comes with the territory of managing people. The best advice I could give you with that is that is you need to either have a group meeting or make time to speak with each individual as soon as possible after you get promoted. Ask them if they have any questions or concerns, let them know that you're the type of person they feel they can come and talk to about any issues they are having, but also make sure you establish the fact that you are now their superior and need to be respected as so.

Its hard since i don't know what kind of work environment you're dealing with or the personality types you work with, but if you're worried there's going to backlash from the people you have to manage, you really have to address it as quickly and professionally as possible. The longer it goes without it, the harder it's going to be for you in the long run.