Yeah, silence is the key to suspicion. He probably doesn't want to be noticed. Therefore, we have a justification for blaming him. also, he was apparently wronged by flippo, and I wouldn't put arson past his arrogance.
If we choose him, we have a perfect motive: revenge.
Who then? Do you have any evidence at all? I doubt it. WE have evidence against no one else BUT DUDEGUY. Logic would lead us to him. We have motive. The guy's used flamebombs before, and knows how to make 'em. There's absolutely nothing supporting you, and all signs point to dudeguy. As far as we're concerned, it was dudeguy.