The idea, of course, was to try and find out if the unicorns were still being hunted, and if so, by what. Whatever Quirrell (Riddle?) was, he'd taken one of Hagrid's crossbow bolts to the chest and lived (?) to tell the tale, but unless they could catch him red-handed, none of the other professors were likely to believe them. If Snape was working with Quirrell, he'd no doubt lie and get them in trouble, and if Snape wasn't working with Quirrell, he probably still wouldn't be any help, because this was Snape they were talking about. Which meant it was, in a best case scenario, their word against one of their professors, and somehow the twins sincerely doubted that that was a conversation with the Headmaster that would go anything resembling well.