Fantastic quote: "A man saw a skull floating picked it up and said, 'You have drowned because you drowned others. But those who have drowned you will be drowned themselves.'"?
I think it is in a Mishna..one of the first chapters or one of the first verses.
That is another quote from Pirkie Avot {Ethics/Wisdom of the Fathers}
This is Chapter 2 Mishnah 6
http://www.chabad.org/library/article_cdo/aid/680285/jewish/Chapter-Two.htm4. He would also say: Make that His will should be your will, so that He should make your will to be as His will. Nullify your will before His will, so that He should nullify the will of others before your will.
Hillel would say: Do not separate yourself from the community. Do not believe in yourself until the day you die. Do not judge your fellow until you have stood in his place. Do not say something that is not readily understood in the belief that it will ultimately be understood [or: Do not say something that ought not to be heard even in the strictest confidence, for ultimately it will be heard]. And do not say "When I free myself of my concerns, I will study,'' for perhaps you will never free yourself.
5. He would also say: A boor cannot be sin-fearing, an ignoramus cannot be pious, a bashful one cannot learn, a short-tempered person cannot teach, nor does anyone who does much business grow wise. In a place where there are no men, strive to be a man.
6. He also saw a skull floating upon the water. Said he to it: Because you drowned others, you were drowned; and those who drowned you, will themselves be drowned.