Single word for attributing inappropriate motives to others' actions?

Is there a single word to describe someone who often attributes inappropriately trivial/cheap motives to another person's actions? I'm thinking along the lines of someone who puts himself briefly in that position and usually assumes the easiest/least flattering motive for other people's behavior. I know "cynical" might be of use here, but I'm wondering if there's a narrower/more specific word to describe this?

For example: Person A and person B both take a test that can be passed or failed. Person A calls person B on the phone in hope of getting the test results only person B (for an irrelevant reason) has access to. Before person B discloses the results, person A asks if the situation is bad, referring to whether they both failed. Person B claims the situation is worse, as a matter of fact, person A failed and person B passed the test (clearly assuming person A will see the situation in the same way). Person A actually thinks this is an objectively better situation and puts person B's success over the comfort of not being worse than them.

"Cynical" may meet your requirements. Also, one of the meanings of "ungenerous," I believe, is willing to ascribe bad motives. (Caveat: I do not have a dictionary at hand at this moment.)