Robert was actually an Agent of the Slender man all along.He knew that stating that All Myths Are True, and asking for a hero for that purpose would encourage some people to actively attack Slendy, which just meant that Slendy got more victims. He didn't want to save anyone.
Robert did lie about the knife, but it wasn't to make himself look good.It was a Gambit on his part to instill a sense of hope and positivity in the bloggers to thus weaken Slender Man and his influence. It makes since Slender Man runs on negativity and fear.
Everything Robert said was true, but he was no match for Redlight in a fight.As the result, he was undone by the latter in the end. The knife now has ever been confiscated or destroyed, Robert then was tortured and had Laser-Guided Amnesia forced onto him. After all, if he gets to go back to his happy life, he can't be seen a martyr.