Pewdiepie reviewed this sometime not too long ago and said the steps to an apology “I’ve made a severe and continuous lapse of my judgment and I don’t expect to be forgiven I’m simply here to apologize”
To be judged by 12 is to go to court and to be judged by a 12 man jury, which all court trials have.
To be carried by 6 is to be dead. It is common for 6 people to carry a coffin (3 on each side) to and from the hearse.
In total the phrase "I'd rather be judged by 12, than carried by 6" means that the person would rather kill someone and go to prison than to, themself, be killed and have to be buried.