11 But listen now to what the Lord says: “You,” said He, “are of your father the devil, and the lusts of your father ye will do.” This is how you are his children, because such are your lusts, not because you are born of him. What are his lusts? “He was a murderer from the beginning.” This it is that explains, “the lusts of your father ye will do.” “You seek to kill me, a man that tells you the truth.” He, too, had ill-will to man, and slew man. For the devil, in his ill-will to man, assuming the guise of a serpent, spoke to the woman, and from the woman instilled his poison into the man.
They died by listening to the devil, whom they would not have listened to had they but listened to the Lord; for man, having his place between Him who created and him who was fallen, ought to have obeyed the Creator, not the deceiver. Therefore “he was a murderer from the beginning.” Look at the kind of murder, brethren. The devil is called a murderer not as armed with a sword, or girded with steel. He came to man, sowed his evil suggestions, and slew him. Think not, then, that you are not a murderer when you persuade your brother to evil.
If you persuade your brother to evil, you slay him. And to let you know that you slay him, listen to the psalm: “The sons of men, whose teeth are spears and arrows, and their tongue a sharp sword.” You, then, “will do the lusts of your father;” and so ye go madly after the flesh, because ye cannot go after the spirit. “He was a murderer from the beginning;” at least in the case of the first of mankind. From the very time that murder [manslaughter] could possibly be committed, he was a murderer [manslayer].
Only from the time that man was made could manslaughter be committed. For man could not be slain unless man was previously made. Therefore, “he was a murderer from the beginning.” And whence a murderer? “And he stood [abode] not in the truth.” Therefore he was in the truth, and fell by not standing in it. And why “stood he not in the truth”? “Because the truth is not in him;” not as in Christ. In such a way is the truth [in Him], that Christ Himself is the Truth. If, then, he had stood in the truth, he would have stood in Christ; but “he abode not in the truth, because there is no truth in him.”
Source: Tractates on the Gospel of John (New Advent)