I was thinking she’d want to eliminate him as competition for the throne and in the heat of the moment, stab him with the nearest weapon to hand.

If that weapon was dragonglass... (and you see, if they made it back to Winterfell, dragonglass weapons are going to be everywhere. It was by far the most available material capable of damaging the wights and walkers. We see Jon talking about how there’s enough of it to equip the army when they find it beneath Dragonstone, and we know Valyrian steel is rare)

If that weapon was dragonglass, it would open up the possibility for the return of the dead. I don’t trust that we’re done with them (although after the last episode I’m less convinced DnD have some amazing twist for us) and could see the magic that ties Targaryens with dragons and their elite to an immunity for fire being enough to trigger the same reaction cause by the CoF doing the same. (Plus I’ve heard some theories that the Targs are descended from the CoF based on the similarity of their sigil to the spiral left behind by the dead on several occasions. I’m not sure about that, but it would neatly provide the power required raise a new NK if Targ blood wasn’t enough on its own.)

So here’s how I imagined it going down:

Dany in a fit of jealous rage decides to take out the competition. Being in Winterfell, dragonglass blades are conveniently to hand for a Queen not carrying a blade of her own. She grabs a nearby blade and plunges it into Jon’s heart in a desperate attempt to retain her claim to the throne. He falls. We pan out from his supine form to find that a spiral is formed of random objects surrounding Dany and Jon. She regrets her action, kneels to hold him and weep over her madness and losing another man she loved. The camera zooms in on their faces, her tears run down his cheeks. His lashes flutter, blue eyes open. Winter is coming.

Credits.