Originally Posted by
GoktimusPrime
I don't see why Leia's death would've been any more or less tormenting for Kylo Ben than Han's. Throughout the film we see Kylo struggling with the decision to do this. Talking about how he can still hear the Light Side calling him. We also see Ben struggling with the final decision, as he had tears streaming down his face and nearly surrendered his light sabre to Han. It felt like his decision really could've gone either way.
And this is similar to previous moments we've seen when a character switches between the Light and Dark Side -- they have an internal moral battle until one side triumphs. With Anakin, we see him watching Palpatine and Windu fighting each other, torn between which side he should support. Ultimately his own greed/selfishness triumphs as he attacks Windu, but we also see Anakin in tears followed by immediate regret as he falls to his knees ("What have I done?!"). Vader also similarly goes through an internal battle, which Luke always knew ("I feel the conflict within you. Let go of your hate."), and while Vader initially denied it, he used his dying words to tell Luke that he was indeed conflicted ("Tell your sister... you were right."). Vader watches the Emperor killing Luke and he becomes tormented over which side to take. But this time he makes the right decision and saves his son, thus returning the the Light Side of the Force (or as Obi-Wan would put it, it was the moment that Anakin Skywalker was resurrected and destroyed Darth Vader).
But yeah, I don't think it really matters which parent Ben killed. Murdering either one was enough to baptise him in the Dark Side of the Force.