Twain.

Be respectful to your superiors, if you have any, also to strangers, and sometimes to others. If a person offends you, and you are in doubt as to whether it was intentional or not, do not resort to extreme measures; simply watch your chance and hit him with a brick. That will be sufficient. If you shall find that he had not intended any offense, come out frankly and confess yourself in the wrong when you struck him; acknowledge it like a man and say you didn’t mean to. Yes, always avoid violence; in this age of charity and kindliness, the time has gone by for such things. Leave dynamite to the low and unrefined.

Read the rest of his essay, "Advice To Youth" here.