The ethics of the power players in Silicon Valley are far worse than those on Wall Street. (I don't know dick about Hollywood other than its reputation.)
Wall Streeters are greedy, but they don't go out of their way to ruin peoples' careers (except in the movies). They'll do a lot of things to get that $10 million bonus, but they don't hold long-term grudges and wreck careers of people who've since flown away from them. A trader will get you fired if it suits his career goals, because he is ruthless, but once you're out of his way, he'll take an active interest in making sure you get a better job afterward. Valley people, when they separate, tend to make it really ugly: lots of gossip, negative references, long-term grudges and other nonsense that wouldn't exist among people who cared more about themselves winning than about other people losing.
Silicon Valley attracts people who want power, and those tend to be worse than those who want money. It also attracts, more than nerds, people who come on specifically to take advantage of nerds. The latter set tend to be horrible, and to rise fast.
Finally, there's the false poverty effect. If you make $10 million in a year, you're rich. If you're a trust-fund kid, but you're only paying yourself a $25,000 salary because that's all your investors will let you take out, you feel more entitled to fuck people over because you're "the little guy" and because, being "scrappy" (or "lean"), you "can't afford" the costs of being ethical.