Evil herself drinks the largest portion of her own poison.

"Let us therefore avoid being ungrateful, not for the sake of others but for our own sakes. When we do wrong, only the least and lightest portion of it flows back upon our neighbour; the worst and, if I may use the term, the densest portion of it stays at home and troubles the owner. My master Attalus used to say: "Evil herself drinks the largest portion of her own poison." The poison which serpents carry for the destruction of others, and secrete without harm to themselves, is not like this poison; for this sort is ruinous to the possessor."

by Lucius Annaeus Seneca, 1st century c. 64 AD.

Letter 81, 22 - Moral letters to Lucilius (Epistulae morales ad Lucilium),  translated by Richard Mott Gummere in 1917.