Just my opinion... There are a lot of publications crying for the prosecution of CEO and whoever else made decisions in these companies. I don't think so.
MF Global was an investment fund. As any investment company, it carried an implied risk. This risk was published in the fund business model: use leverage to buy short term government debt, mostly European and hold to maturity. What can go wrong if all governments (with small exception of Greece) paid all transhes in full and in time? Leverage. MF Global had to use debt securities as a collateral when taking loans. When those securities lost value because of another debt crisis, creditors required more collateral. It was classic margin call situation. Definitely, MF Global management made mistakes. Maybe somebody stepped outside the law, but only serious law enforcement investigation can prove it. So far it didn't. Investors lost money, but that's exactly the risk they accepted when investing in MF Global. I understand emotional aspect and lust for blood, but law is not about emotions. My advice to investors: research companies carefully.
JP Morgan (JPM) case is more complicated. The only reason they are not prosecuted now is repeal of Glass Steagall act. Banks, especially big, "too big to fail" banks, should not be allowed to trade like hedge funds. But they are allowed now and they are doing it. So, what JPM did was probably legal, but shouldn't be. Because if too big to fail bank fails, taxpayers will pay. You can shout, you can say "no more bailouts" but in real life, when there is a choice between a bailout of banksters and financial panic, any responsible government chooses bailout. That's why Glass Steagall act existed, that's why it should be restored in some form.
There is no evidence so far that JPM or some people there broke existing law. It means that nobody goes to jail, now. It also means that law must be changed. US (and, by extension, World) financial system should be protected from careless speculators.
Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours.