Captain Gutt is a pirate from the Ice Age that's willing to steal everybody's food. Thanks to Lord Garmadon he's been transported to the future to do his bidding at first he didn't want to but then Lord Garmadon said if he didn't then he would trap him trap him in an endless void of time where he will never steal anymore food again. And he's willing to destroy Grievous.
Lord Garmadon is his new master. He was forced to do his bidding because if he didn't then he would send him in an endless void of time.
Grievous was the first one to attack which made him hate Grievous and made him his mortal enemy.