"I didn't want the spell to be revealed because of the Arthur/Gwen, but rather for Gwen's peace of mind."
Oh yes I get that... but if she knew now then she'd have told Arthur and his love might have looked conditional (like, 'okay I forgiver her now that I know it was a spell'). I still hope that Gwen will learn later that she was always innocent....
I'm just happy that Arthur's love is proven to be unconditional.

Also, I was wondering... had Merlin ever really killed in cold blood before? Because whats-his-name/Agravaine had surrendered, hadn't he?, when Merlin offed him, which seemed like amazingly bad-ass for Merlin! I was really impressed.