Why would biblical God kill satan? You never read biblical text? Biblical Satan is translated from meaning adversary and it is what God sends to oppose what people do.

Precisely

And the same reason we still don't have gun control or the abolition of abortion by the Republicans.

Another way of looking at it is. When they invented God, they made a few mistakes in the script, they could not do a rewrite because they had got a lot of it in the can already, so then they had to invent the devil to try and get the plot to make sense. (Did not work, complete turkey.)

And yet that's exactly how the natural world is. It never kills off evil, and the show goes on.

Have your son tortured and killed, but let the
bad guy keep running around.
Kinda like how Batman never kills his villains no matter how many people die.