This is a forensic speech in prosecution of Eratosthenes, a member of the The Thirty. It is not contested that Eratosthenes killed Polemarchus. Eratosthenes pleads that he was following orders from the Thirty. And declares that he had opposed the death sentence when the Thirty considered the matter.
καὶ μὴν οὐδὲ τοῦτο εἰκὸς αὐτῷ πιστεύειν, εἴπερ ἀληθῆ λέγει φάσκων ἀντειπεῖν, ὡς αὐτῷ προσετάχθη.
Here is my trial translation:
And indeed [it is] not plausible to trust in the justice of his case, even if he says truly that he opposed [the death sentence], and that it was ordered of him [to kill Polemarchus].
I am unsure about the bracketed bits, and about my understanding of the logic of this sentence.