Aye indeed - wait and see and have faith! Everyone is allowed a moment of weakness when their fears get the better of them - especially when they are in the grip of a terrifying and fatal supernatural illness that comes complete with fever, delusions and hallucinations! Dean does believe in Sam's inherent goodness, but this hallucination stemmed from the same fears that Sam has expressed in the past: that being a good person might not be enough to prevent some kind of change happening. After finding out about Sam's lies and use of his power, Dean can be forgiven for having his faith in Sam's ability to resist a little shaken, in a weak moment.