Testing the green liquid
As soon as Edward got the order, he knew he had no choice but to comply. The fact that they had sent the message directly to him and it was a summon too, meant that he would have no choice. If he was to ignore the order, then there would be serious consequences for doing so.
Before leaving, he decided that he would inform Peter of his departure but didn't tell him the exact reasoning for his leaving, only for the rest of them to wait for his return.
"And if you don't return back before the execution?" Peter asked.
There was a slight pause as Edward was thinking of how to answer this, as there was a good chance this might happen. Edward wasn't naive enough to believe that they wouldn't have some spies and won't be blind enough to not notice the tower lighting up.
If he didn't give a good enough reason for these things happening, then they could very well keep him at the king's castle.