Merlin's reasons for lying to Arthur, which were the same reasons he needed to kill Mordred were not pure.
Haven't you seen minority report?
Every time the Dragon said "Kill Mordred" or "Kill Morganna" and Merlin said "no", that was about purity.
killing innocent people for crimes they have yet to committed is wrong.
Even if they're asshoels.
He was killing for vague and uncertain reasons.
I don't think Arthur is listening any less or more than he has before, but yes that was a superb moment of transition Merlin fumbled.
And what the hell was that "Would you risk your life to save me?" clap trap? Arthur, Gwen and Morganna cowboyed up and rode out to Merlin's village and fought off Julian Bashir and then something like that happened every other week too.
I want big multi episode stories.
They can do it.
All these bottle episodes are for poop.