Merlin sincerely enjoys the killing, the torture, it comes with having been born and raised by the Demon Clan.
However, the only moments she can say she has been truly happy is when she's in Arthur's arms, not even when Sis-sis and Maliodas used to take care of her had she felt like this.
Her adorable, kind and brave Arthur. Who did not look at her with fear or disgust in his eyes, not even with lust, but with love. He took all of her and accepted it without a second chance, saying that Merlin was still Merlin.
Maybe this was the feeling that humans called love.
But whatever it was, it changed Merlin- for good.
Which got them into the mess of the situation they are right now.
Merlin simply to help others to be happy with the one they love, just like she was with Arthur. So, she found a spell to anchor Elaine's soul to Ban's, making it so that even after killing Melascula the fairy won't disappear. That way they can fight against the Ten Commandments without fear of loosing one of them.
The problem lies in the fact that even though Ban has the fountain of youth running through his blood it was not strong enough to keep Elaine alive and out of Melascula's curse. That's where King entered in action.
His blood relationship with Elaine, plus his power as the King of Fairies, would be enough to sustained Merlin's spell. Of course, King accepted immediately- anything to save his little sister. Even if it meant loosing the man he loved forever.
Though, what the three did not know is that once the spell entered into action their souls would be interlaced forever. They would be able to read each other's heart clearly, know where the other is at all time, share memories and feelings.
You can guess the rest easily...
As soon as Merlin's spell entered into action a strong wave of regret, happiness, sorrow, relief, guilt and heartbreak flood Elaine and Ban.
King's eyes widened when he noticed what had jus happened:
His secret was out to the last persons he wanted to find out.
Without even thinking he flew away as fast as he could, leaving the two most important people in his life speechless and more confused than ever.
"Did that just happen?" Elaine asked, to no one in particular, in disbelief.
"Dammit!" cursed Ban, angry at the whole situation.
The bandit would be lying if he said he felt nothing for the sing of sloth, he was way similar to Elaine not to- but also different enough that made the King of Fairies unique and a special treasure for the thief.
But he had kept quiet and controlled his desires not wanting to hurt his dear Elaine. However, somewhere in his mind he kept desiring being able to have both of them by his side. He was that greedy after all.
He kept imagining the three of them traveling the world, while fighting against the Ten Commandments. Then they will finally go back to the forest and live the rest of their lives happily ever after: taking care of the fairy people (though, that will mostly be Elaine and King) and drinking the delicious ale from the berries that grow there (that will surely be Ban).
What the fox did not notice was that through the shared link the three of them now shared, the fairies saw those dreams. But what he did felt was the desire and happiness coming from King and the curiosity from Elaine.
"You men are truly exasperant and idiotic! How the two of you survived without me so long, I'll never know" sighed the guardian maiden.
"What's going on?" Arthur asked his girlfriend, as he watched Elaine and band ran after King, stranged by the whole scene that unfolded in front of you.
"My job is done here" was all that Merlin replied, before dragging the blond man/boy inside for some loving.
Humans and their love...