He supports this claim by asking Polus what it is about certain things that makes them admirable. Together they come to the conclusion that what is admirable is defined as what is good and pleasurable; consequently, what is shameful is that which is painful and evil. Using this definition, Socrates questions whether or not it is more painful for a person who acts unjustly than for one who suffers injustice. Polus quickly denies this, and so Socrates is able to say that doing what is unjust is more shameful and evil than suffering from it.
Personally, I find that Socrates’ conclusion that committing injustice is worse to be the right one. Seeing as how Socrates was oftentimes more concerned with the wellbeing of the soul over the body, it makes sense that he would decide that acting in an unjust way would do more damage to an individual than having someone act that way towards them.