The sinner sins against himself. The wrong-doer wrongs himself by making himself evil.

[Ὁ ἁμαρτάνων ἑαυτῷ ἁμαρτάνει: ὁ ἀδικῶν ἑαυτὸν ἀδικεῖ, ἑαυτὸν, ἑαυτὸν κακὸν ποιῶν.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 9, ch. 4 (9.4) (AD 161-180) [tr. Hutcheson/Chrystal (1902)]
    (Source)

(Source (Greek)). Alternate translations:

He that sinneth, sinneth unto himself. He that is unjust, hurts himself, in that he makes himself worse than he was before.
[tr. Casaubon (1634)]

He that commits a Fault Abroad , is a Trespasser at Home; And he that injures his Neighbour, hurts himself , for to make himself an ill Man is a shrew'd Michief.
[tr. Collier (1701)]

He who does wrong, does a wrong to himself. He who is injurious, does evil to himself, by making himself evil.
[tr. Hutcheson/Moor (1742)]

He that commits a crime, is guilty of an offence against his own interest, and he that acts unjustly, injures himself: for to make himself a bad man, is an essential injury.
[tr. Graves (1792)]

He who does wrong does wrong against himself. He who acts unjustly acts unjustly to himself, because he makes himself bad.
[tr. Long (1862)]

He that commits a fault abroad is a trespasser at home; and he that injures his neighbour, hurts himself, for to make himself an evil man is a great mischief.
[tr. Collier/Zimmern (1887)]

He who sins, sins against himself; he who does wrong, wrongs himself, making himself evil.
[tr. Rendall (1898)]

He that does wrong, does wrong to himself. The unjust man is unjust to himself, for he makes himself bad.
[tr. Haines (Loeb) (1916)]

Whosoever does wrong, wrongs himself; whosoever does injustice, does it to himself, making himself evil.
[tr. Farquharson (1944)]

The sinner sins against himself; the wrongdoer wrongs himself, becoming the worse by his own action.
[tr. Staniforth (1964)]

Whoever does wrong, wrongs himself; whosever acts unjustly, acts unjustly toward himself, because he makes himself bad.
[tr. Hard (1997 ed., 2011 ed.)]

To do harm is to do yourself harm. To do an injustice is to do yourself an injustice -- it degrades you.
[tr. Hays (2003)]

The sinner sins against himself: the wrongdoer wrongs himself, by making himself morally bad.
[tr. Hammond (2006)]

He who acts wrongly harms himself. If a person commits an injustice, he acts badly toward himself, thus making himself bad.
[tr. Needleman/Piazza (2008)]