Sen. Carl Levin’s series of hearings on Wall Street’s collapse reached their dramatic climax today, as Goldman Sachs executives sat in for their roles as national punching bags. Everybody from Levin to Sen. John McCain got in on the action, showing outrage at Goldman’s ability to make bundles of cash by betting against the same securities it sold to investors. “You have less oversight than a pit boss in Las Vegas!” raged Sen. Claire McCaskill. It was a nice dart, and one that gets us to the point: the current standoff over how Washington will rewrite the rules for Wall Street’s behavior. But Levin’s three previous hearings are far more instructive than today’s Goldman bashing (as satisfying and appropriate as it may be). The first hearing revealed evidence that Washington Mutual knew in real time that its subprime mortgages were often fraudulent. In the next, federal regulators failed to answer reasonably for why they refused to rein in WaMu’s lending behavior, despite evidence of that fraud. The third hearing showed execs at credit-rating agencies — at Moody’s and Standard and Poor’s — cajoled staff to give securities backed by plainly shoddy loans high ratings nonetheless. The financial reform debate is complicated, to be sure. But Levin simplified things well this morning when he said there’s a thread connecting all of this behavior: “That thread is unbridled greed and the absence of a cop on the beat to control it.” But another thread has run through these dismal hearings: Behind the shoddy loans in question are struggling borrowers who were kindling for Wall Street’s bonfire.