Why did this character become a Dark One?

Question

In the first half of Season 5,

Emma uses the part of Excalibur that Merlin was tethered to to transfer Merlin's magic to Killian.

According to

Merlin, Nimue became the Dark One because she used the magic from the Grail to kill somebody. Merlin himself had never killed anyone and so his magic hadn't turned dark, but when Killian got it he became another Dark One.

Did they explain why, contrary to what one might expect,

Killian immediately became another Dark One instead of a "Light One" or whatever Merlin had been?

One possible explanation could be that

Killian had already killed people at that point

and that the person's history is what counts, but by that logic

Emma shouldn't have become a Dark One as she hadn't killed anybody.

Answer 1

Excalibur and the Dark One Dagger are neither inherently good nor evil, just incredibly powerful items as they were forged from the Holy Grail. Note that

Nimue became a Dark One after drinking from the Grail but before the dagger was created, by crushing the heart of Vortigan, which is very dark magic in that universe, while Merlin remained pure after drinking from the Grail. Being neither inherently good nor evil is also how a full Excalibur can be used to destroy all the light OR all the darkness in the world, as either side tries to use the blade in the fifth season.

One of the central themes of Once is the never-ending battle of good (light) and evil (dark), and much of the storyline is driven by characters moving away from their fairytale extreme, or in few cases crossing it entirely. This is why

Emma is not instantaneously turned into the Dark One after being bound to the dagger, as she is still in her "good" extreme as unlike most Dark Ones, she did not commit a mortal sin to acquire the dagger and the magic imbued in it.

However, their is one significant difference between the otherwise equal Dagger and half-Excalibur:

The Dagger has the soul of Nimue stuck to it. Thus, Nimue is able to appear to whomever is bound to the Dagger (as herself or another Dark One form), and start tempting them towards the Darkness. Emma actually resists for some time as she is very pure due to all the darkness being removed from her as unborn child (although she still blasted Cruella off that cliff before all this, but I digress), but even she admits to Regina before Excalibur is reforged that the darkness is very alluring, as we have seen through her actions (notably controlling Henry's crush with her heart) prior to then. When Killian is about to die, Emma finally gives in to the Darkness Nimue has been urging her towards, using its power to untether Merlin and tether Killian. This makes her a Dark One in action (although the remainder of the season shows she is still working to end the darkness, just in a more evil way than typical). Killian on the other hand, does not have the resolve that Emma does, as he tells her throughout the season, and gives in to the darkness surrounding the magic saving his life immediately, becoming a Dark One on the spot.