Also fun fact: this isn't the first time that Ford had Han killed off either. In The Empire Strikes Back, Ford wasn't convinced that he'd come back for Episode VI, but Lucas really wanted him to return. So they wrote in the scene where Han would be frozen in carbonite. If Ford decided to not return, they would simply say that he didn't survive the hibernation. If Ford changed his mind, then they would write some side plot to somehow retrieve Han. Ford of course was convinced to return for the third installment, hence why the movie starts off with Han's rescue. I think this time round, Abrams didn't want to do go through that whole rigmarole, because even if they convinced Ford to come back for Ep VIII, the first act would be yetanother let's-save-Han story. So Abrams probably just went, "Yeah, fine. We'll kill Han."

Storywise, this scene was better set up for in a deleted scene. Leader Snoke and Kylo Ren are fully aware that Darth Vader reverted to becoming Anakin Skywalker, returning to the Light Side and killing the Emperor, saving Luke etc. Ren would've grown up hearing this story from his Uncle Luke no doubt. However, Snoke taught Ren/Ben that this was a single moment of weakness in who was otherwise the ultimate Dark Lord of the Sith. And in order to ensure that Ren would "immunise" himself from Vader's weakness, his 'baptism' into the Dark Side would need to be killing off Han Solo. Thus severing the possibility of his love for a family member becoming his "weakness" in heeding the call of the Light, as happened to his grandfather.

Of course, this weakness would still exist in Leia... so does this mean that Ren wants to kill her next? One might argue that it would exist in Luke as well, since he is his uncle.

Killing off loved ones seems to be a common way of sealing a Jedi's fate into the Dark Side. Anakin was led to believe that he killed Padmé* and the existence of the twins was hidden from him; so as far as he knew, he killed his wife. This drove him into the Dark Side until his encounter with Luke reignited the conflict inside him, ultimately allowing him to redeem himself to the Light.

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*IMO Anakin did kill Padmé. I don't buy this whole, "she died from a broken heart" rubbish. To me, the Force Choke that Anakin used inflicted some sort of injury that was undetectable to the Kallidahin doctors.