Hung out to dry?

So I was reading Acts 12 today and came across the story of Peter being saved from prison and certain death by an angel of the Lord. The angel basically just supernaturally releases him from his chains and leads him out of the prison past the numerous guards assigned specifically to him. Okay so that story sounds crazy, but I have no problems just believing that it happened. My question though is the aftermath of it all. So Herod is pissed and cross examines all of the guards. They obviously have no flippin clue as to what happened, but that doesn't matter to Herod. He simply orders that they get executed. So let me get this straight. The guards are simply doing their job and as a reward they got executed for what God clearly did through the angel. So did God hang the guards out to dry? Were they being punished for holding Peter captive - regardless of their personal feelings over it? I am not claiming that God is unjust, but when reading that I think it pretty much sucks for the guards. The only explanation I can think of is that they are guilty for keeping Peter in prison so their death is their punishment. Any thoughts on this???