The first time Killian Jones watched the Dark One disappear, he was filled with one purpose: revenge. After all, that was one of two things he fought for in life. His Milah had died at the hands of the Crocodile and he would willingly wait for centuries on a bloody island so he could avenge her.
Whatever possessed him to come back to the Enchanted Forest, he had no idea. All that he knew was it had led him to meet her: Emma Swan. She was a kindred spirit, as he realized, when they set out to climb a beanstalk.
After that they would face many things together: Neverland, time travel, villains, all the while fighting side by side.
And he loved her, so much that it blotted out his thirst for revenge.
But now, he was staring into the face of the Dark One yet again. This time it wasn't a crocodile looking coward. It was Emma. His Emma who had become the one thing he wished for so long to kill.
The second time Killian Jones watched the Dark One disappear, he was filled with a purpose: love. He waited three hundred years to kill the Dark One and he would willingly fight far longer for the woman he loved.