I thought he walked out on the match because he was upset that the computer DIDN'T behave like a human. Huh. Weird, these narratives.
He didn't walk out. He played all six games in the match and lost the match in the last game after he played a opening that was know as losing by everyone after a piece sacrifice (it was speculated that Kasparov was gambling that deep blue wouldn't make the sacrifice and then he will have the upper hand). Deep blue played pretty much "like a machine" except for a few moves that where considered very human because the…
He wasn't being a sore loser and there was a reasonable amount of agreement among other chess professionals (engine authors and players) that his claims were true.
Keep in mind as well that when he asked for a rematch with more stringent scrutiny in place, IBM declined and nearly immediately dismantled Deep Blue.
The unfortunate thing is that kasparov would ALWAYS have looked like a 'sore loser' unless IBM decided to implicate themselves of wrongdoing. Kasparov being silent also does a disservice to the community, particularly since he was neither the first or only person to point out the inconsistencies in Deep Blue's play.
Keep in mind as well that the 'real circumstances' make it so both parties responses were rationally founded within the context of the available information at the time.