In Hitchcock's Strangers On A Train when Bruno visits Guy after murdering his wife the latter wants to go the police. Bruno stops him, saying that he will be arrested for involvement, i.e. they won't believe that it was Bruno's initiative. Guy later says the same to his fiancee, when she finds out.
At the very end Guy is released and no charges are pressed against him, while his situation is exactly the same, the only difference being the fact that Bruno wanted to set him up using the lighter. However if Guy was involved in the murders and Bruno wanted to frame him as being the actual killer (for whatever reason) it would look exactly the same for the police.
Based on the above why the police believed Guy, if it seems that his cooperation with Bruno and later disagreement was much more plausible than Bruno killing on his own and then trying to set up Guy?