What does this mean No better friend no worst enemy?

Answer

That quote is the lifeline for the US Marines. With a Marine as your friend, you have a person who will kill to protect you. With a Marine as your enemy, you have a person who will kill you.

The history behind that quote.

The earliest origin of this statement is attributed to the the epitaph of Lucius Cornelius Sulla, famous general and legendary dictator of ancient Rome. His original Words...

"No friend ever served me, and no enemy ever wronged me, whom I have not repaid in full."

• His self-made epitaph, as quoted in Heroes of History : A Brief History of Civilization from Ancient Times to the Dawn of the Modern Age (2001) by Will Durant; variant translation: "...nor enemy harmed me"

No better friend: someone who will kill to protect you; someone who will give their life for you; someone who will be right there in your times of trouble and, thinking of you and your problems, will sacrifice so that you might get through your problem.

No worst enemy: someone who will not hesitate to destroy you even if that means destroying himself in the process; someone who will chase you down to the ends of the earth to extract revenge; someone who will cause you blood and tears and smile when it happens.