If one pool controls more then 50% of the network it can fake transaction because more then the half of the miners would validate the fake transaction. They could create any balance on there account they want. And the real value of the coin would drop to 0.
That is almost completely wrong
Miners cannot fake transactions or alter the balances of accounts.
What they can do is selectively decide which transactions to process.
The main risk of a 51% attack is the ability to reverse transactions by double-spending.
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power