Kylo Ren didn't activate the lightsaber...
…during his confrontation with Han Solo. They’re shown both holding on to Ren’s lightsaber, as if fighting for control of it, much like struggling with a loaded gun and having it go off.
The simplest possibility was mere accident prompted it to activate the way it did. However, another possibility is that Han was attempting some kind of gambit on Ren and deliberately sacrificed his life in the process. By allowing Kylo Ren to kill him, after Han promised he would do anything to help him, it may create a bond of trust that balking at his request might have destroyed, in addition to leaving Kylo a chance at redemption by taking it out of his hands since Kylo killing Han willingly would have ensured his complete and total allegiance to the Dark Side.
Dovetailing with this is Han’s well-known reputation as a smuggler and a gambler - two professions that require the ability to accurately read people. He goes into the confrontation in all earnestness, but as soon as Kylo Ren shows his face and seemingly goes along with him, Han knows he’s lying. His gambit failed, he is left with no choice but the aforementioned Heroic Sacrifice
In future episodes, we may see Ren having visions or hallucinations, since a Force Ghost is highly unlikely, of his father, Han. It may even act as a Good Angel, Bad Angel scenario, since the actor is confirmed to return in Episode VIII.
And if they want to reference the knight of the republic games, they could make this a great moment for Han. In the original Kot OR, Malak attempts to have Bastila kill someone she loves, in this case a redeemed Revan, to extinguish the light within her. If Luke happened to tell Han the old Jedi legend of the Prodigal Knight, he would have known about that and possibly sacrificed himself to prevent his son from being able to go through with performing the same kind of atrocity, thus leaving a spark within Ben for Luke and Rey to try to ignite.]]
^To the anon that submitted the above: I’m in agreement that Kylo will more than likely suffer from some sort of hallucination in Episode VIII, and that Han’s death served a much greater purpose than simple narrative tension (and Ford’s desire to leave the franchise).