What would happen if a mining pool admins having majority of hash power turns evil?
After reading two papers: https://blog.ethereum.org/2015/01/28/p-epsilon-attack/ and https://www.cs.cornell.edu/%7Eie53/publications/btcProcFC.pdf I conclude, that the neither ETH nor BTC (and most if not all any other crypto currencies) are secure against such attacks. It is also not secure against a so-called Schelling scheme, i.e. attack made not by the party that controls a big pool of miners, but simply a party that has enough coins to bribe miners into mining his own double-spending block. If the attacker is able to bribe majority of the miners, such an attack can be zero-cost for the attacker. 

Specifically, "The P + epsilon Attack" paper concludes:


  Of course, in practice, proof of work has survived despite this flaw,
  and indeed it may continue to survive for a long time still; it may
  just be the case that there’s a high enough degree of altruism that
  attackers are not actually 100% convinced that they will succeed – but
  then, if we are allowed to rely on altruism, naive proof of stake
  works fine too. Hence, Schelling schemes too may well simply end up
  working in practice, even if they are not perfectly sound in theory.


I hate to be a bearer of bad news, but the world with affordable, decentralized and deregulated finance services Ethereum and other cryptcurrencies bring might not be as secure, as we might want think. 



Another sad update in this article: Decentralization in Bitcoin and Ethereum Networks, 11 Jan 2018