If hackers was able to gain control maybe at least half of the mining process of a certain block, from what I've read, they can form those included transaction and create forks which real tx aren't reflected, through that they can double spend bitcoin.
As has been explained already in this thread, having 51% of the hash rate does
not allow an attacker to create bitcoin out of thin air, double the amount of bitcoin they own, give themselves bitcoin out of nothing, or anything of that nature. It also does not allow them to help themselves to anyone else's bitcoin. It allows them to double spend their own transactions only. Note that if a bitcoin was successfully double spent, then the original transaction becomes invalid and is erased from the blockchain, leaving only one valid transaction. The bitcoin being double spent is not actually "doubled".