Presidential candidates from Bernie Sanders to Donald Trump have condemned their tax-dodging. Investors have deplored their outlandish fees and poor returns. Showtime has made one of them the arch-villain of its new series “Billions.” Hedge fund managers may be feeling put-upon these days. The rest of us should be glad they’re finally getting the scrutiny they deserve, for a range of shadowy practices that cannibalize the real economy and aggravate the problem of extreme and growing economic inequality — and for the tax ploys and rigged rules that make it possible. What is a hedge fund? In theory, it’s a...