οὐκ ἔστ' ἀνυστὸν τόνδε σοὶ κατακτανεῖν

From LSJ

ἀλλὰ σὺ μὲν νῦν στῆθι καὶ ἄμπνυε → but you, stop now and catch your breath | but do thou now stand, and get thy breath

Source

English > Greek (Woodhouse Quotes Reversed)

it is not permissible for you to kill him