I think this hurts his credibility somewhat. It's a better story to think he joined with no preconceived notions and the evidence turned him. Now it appears he had a conclusion and went to prove his assumptions to be true.
But why does this not increase Snowden's credibility? He went undercover to gather evidence of malfeasance. How does that not turn him from a leaker into an investigative journalist getting an inside scoop?
Like the GP post said, the intentions or motivations (emotional or intellectual) are of little concern as long as the scientist changes their views/theories based on experimentation. I could be emotionally invested in what I believe to be a life-saving cure for cancer, and I'll be emotionally pre-disposed to see the medical trials prove that, but that won't mean I'll fix the results to prove that.