Karpeles does not have a serious history of fraud. We don't know yet what happened with MtGox. Wait and see.
What gave you that idea? There's no police investigation ongoing in Japan, or anywhere else for that matter. Karpeles is looking at Japanese bankruptcy proceedings, he is not facing criminal charges. He was invited to testify in Texas in light of probable criminal wrongdoing, but he turned down the offer. The only major "investigation" of Karpeles is happening on Reddit and HN.
There are various new articles about the Tokyo Metropolitan Police and Mt.gox "working together", but it sounds like you are right that the police are not investigating Karpeles.
Consider: Richard Nixon was never convicted. Does that mean I can't/shouldn't call him a criminal piece of shit?
The current investigation will hopefully surface whether the debacle was the result of genuine mistakes on the part of MtGox, or the result of wrongdoings. I'm personally reserving my judgment until we have more trustable information.
There is a lot of hard evidence:
1. 800k+ bitcoin that are gone
2. Public statements by Karpeles that all was ok on numerous occasions
3. Frozen withdrawals for months that hinted at liquidity problems that were denied (which is a crime)
4. A leaked crisis strategy draft, later admitted by Karpeles as a legit internal document, that showed MtGox knew at least a month before d-day that they were in trouble and trading insolvent.
5. The malleability bug excuse used as a cover to liquidity problems
6. Timeline of withdrawal problem suggests earlier knowledge and inaction
What is in doubt is the extent of the coverup, who else was involved and if Karpeles really was behind a plan to ease the liquidity burden by settling his goxbux debt by hyperinflating his remaining bitcoin holding value, and "printing" bitcoin by buying on the market in exchange for GOX USD - which could then be smuggled to another (legit) exchange in return for real USD that could be used to pay back the deflated goxbux.