Set thou a wicked man over him; i.e. to judge him (see Psalms 109:7). A persecutor deserves to be himself persecuted, an oppressor to be himself oppressed. "Nec lex justior ulla est, Quam necis artifices arte periresua."
And let Satan stand at his right hand; rather, an adversary, or an accuser. In courts of justice the accuser stood at the accused person's right hand.