No. Having 51% of the hashpower should not be illegal. Attempting to use that power to defraud someone should be, and probably already is. At least from a common sense point of view, issuing a transaction that pays Bob, and then issuing another transaction that sends the same coins to yourself instead of Bob, is a pretty unambiguous attempt to defraud. It's legality does not need to depend on you having 51%.

(Some people argue that double-spending can be a normal part of a payment protocol, eg for micro-payments. However, although those protocol may involve creating conflicting transactions, so far as I know only one of those transactions actually needs to get broadcast to the network. Still, it might be best to avoid naive legislation.)