He wants to nuke the Long-15 regardless of who you side with. He plans to bleed out NCR by destroying their most vital trade route and cutting them off from the Mojave, which in turn also stops the Legion in its tracks and halts their momentum - bringing them to an early collapse (Because he believes that the Legion will collapse when it runs out of steam, i.e if they defeated NCR and reached the sea, the same would happen), and then letting the Mojave and the rest of "The West" stir and fly a new flag in the wake of both factions dying out. He doesn't blame the Courier for the Divide's destruction - he views the Courier as carrying an unintentional life lesson/message - that one individual can change the course of history, 'make or break a nation', as the Courier did at the Divide (By killing off what he thought was going to become a new nation). Then you get into the metaphor stuff which is a bit convoluted with his choice of the Long 15 being perfect because it's the way Couriers travel through NCR (as you did extensively) and it's "your home" for that reason, so there's a sense of cosmic/poetic justice in it. The only personal animosity he has toward the player is that he thinks you're careless, basically the archetype of the Fallout protagonist - wander into places, change the course of their history and then leave without any investment or reflection on it. Which is something you can prove him wrong on in your final confrontation via Faction Rep.