If guilt over murdering his own son are insufficient, I doubt some accurate statements (”insults”) from an anonymous person on the Internet are going to do the trick.
It’s objectively true that he participated in the murder of his child - that much is clear, and must be articulated in any attempt to bring him to repentance. But, calling him a “vile agent of murder” is taking it a bit too far.
How do you think he’d respond to this: “You’re a vile agent of murder for what you did to your son. Now accept Jesus Christ as your personal Lord and Savior.”