Let's get some things straight.
I see a lot of people going “awww, but what she did was justified, so why is that dark?” - and yes, Cora dying for her crimes IS justified.
- The problem is that that’s NOT why Snow did it. Snow did it out of revenge. Not to save someone. Not to protect. But out of revenge. So while the said person might have “deserved it”, the intent is what matters. And Snow intent was not about saving Rumple(she actually implied she doe snot give a fuck whether they both get killed, in fact she thought that would be better outcome), not about punishing Cora for her deeds, not about protecting someone from Cora. Her intent was simply that - killing Cora because she wanted to.
- The problem also is on HOW Snow did it. Snow chose one of the most darkest way to achieve her goal, pretty much almost surpassing Rumplestiltskin himself in manipulativeness and coldness. She twisted someone’s good emotions - love, care, loneliness into a weapon for herself without a second thought through the whole con, clearly knowing what is going to happen.
And now Snow is realizing what she did and is horrified enough to be suicidal. She has seen this with Regina, she has known this happened with Rumple and she wants to die because of what she is becoming - it basically has destroyed her identity, everything she believed she was.
And at the same time she is suicidal NOT because of what she did to someone, but because of what it does to herself. Snow wanted to die not because she hurt someone, but because her actions are hurting herself, psychologically and slow realization of that is doing nothing but deepening the trauma.
And all of this self-loathing, remorse and depression does not always lead to redemption. It makes a person unpredictable, more prone to giving in to the worst parts of themselves, making them shut off from people they care about, making them hurt the people they care about.
And as it goes on, Snow will either get consumed by her loathing and darkness(like Regina, her antithesis and parallel, was) or will accept it and start to justify it(like Rumplestiltskin, yet another one of parallel characters to her, did). In either way, it leads to more darkness and despair.
That is why its dark. And that is why she has been suicidal - through her actions, she is becoming into someone she never thought she had in her.