Even at Competitive REL, you should have been locked into this one from the beginning. The same would occur at Regular REL (Friday Night Magic), but judges at Regular can be a little lax on letting people take back their choices.
The moment you named a creature, you declared your intent to let the trigger from Dictate of Erebos resolve.
You should immediately follow through with its resolution and sacrifice a creature. You will not have a chance to make other choices until after the spell has resolved. In other words, you should never have even had the opportunity to activate Undercity Informer before the trigger resolved.
How do I know this?
I once played Foundry Champion at a Pro Tour Qualifier sealed event in Dallas/Fort Worth. My opponent asked who I was targeting with the enter the battlefield trigger. I chose a creature. He then tried to counter my Foundry Champion. I called a judge, and the judge ruled that because my opponent prompted me to make a choice that could only be made after the creature had entered the battlefield, and because I made the choice, that he had declared his intent to let the creature enter and I had accepted. Therefore, it could not be countered, the triggered ability sits on the stack, and my opponent has priority.
Article by Scott Marshall
Read this. It will not only improve your game, but it will make you a better communicator and therefore more fun to play with.
Most situations involving shortcuts are judged by who has what to gain from making ambiguous choices. In your case, the state of the game was ambiguous. You thought you were making the choice before resolution. We assume that your opponent knows the rules, and thought you were making the choice during resolution. The ambiguity you created caused your opponent to give away information: he had nothing to do before resolution. Therefore, you cannot change your choice. You have gained information that you wouldn't normally have (however minor it may be), and so the game cannot be reverted.