A really rich kid may just do it just for the lulz. A state may do in order to fight tax evasion, money laundering or drug sells. A bank or somebody like PayPal

A really rich kid may just do it just for the lulz. A state may do in order to fight tax evasion, money laundering or drug sells. A bank or somebody like PayPal may do it to get rid of a competitor. And destroying Bitcoin was only a random example, somebody could as well blackmail you to pay a high fee to get your transactions processed. Or they could just double-spend coins for financial gains. There is a endless number of scenarios. But actually this is all not really important. One of the main intentions of Bitcoin is to remove the need to trust a bank or a state. With more than 50 % of the hash rate in more or less a single hand you now have to trust a random guy on the Internet. And even if the mining pool owner and all the miners are perfectly good people, there is now still a single target for an attacker. If I understand the internals of a mining pool correctly, there is a central server collecting transactions and handing out work items to the miners. If somebody gains control over such a system, he may be able to do bad things without anybody even noticing for some time.