But... isn't Herod the one who made the choice to kill those children?
Yes, it was Herod's decision to do that. But the reason he did it is what's important. He did it because the Magi had tipped him off about a rival king having been born. The "star" is the reason that Herod came to learn of one destined to become king.
And how do you know that he wouldn't have done something similar anyway, with or without the star?
What reason would he have had for doing this if it hadn't been for the star? It was due to the star that Herod came to know about the birth of this "king".
And if you follow your own line of reasoning far back enough, wouldn't that make God responsible for all the world's evil since He created the angel who became Satan in the first place?
No, it wouldn't. Just as you said, the angel who became Satan wasn't created corrupt. He became corrupt on his own. God isn't responsible for OUR actions. It's we ourselves who are responsible for what we do.
Personally, I think it's silly to point at everything in the world as being part of a Satanic plot, and it seems a bit far-reaching to try and link the choice some evil human [Herod] made with Satan's Elaborate (and I do mean elaborate) Scheme. That kind of thinking tends to make a person paranoid, and it also removes the element of personal responsibility.
I take you to mean that the idea that Satan had planned to use Herod as an agent to kill the child Jesus is not reasonable. I personally don't find such an idea to be so unbelievable. So why do you? Care to explain?
.