He couldn't kill Canelo, he won't land cleanly enough on Duran to kill him either. this ain't Lemieux .
Problem is that Golovkin bases his game off his jab and footwork and likes a tactical fight. Duran likes guys trying to pressure him and is great at turning their aggression against them. He likes guys getting wild and off balance and that's just not Golovkin. Golovkin can fight smart and at range and ought to be able to use his size and technical approach to pile up a ton of points. He's a good deal bigger and more powerful than Duran, who shouldn't really even be at this weight, especially not under modern rehydration rules.
I like GGG here, but Duran would keep him honest and make him take a little heat off those clubbing shots in order to land enough to earn a decision.
One reason and one reason only does GGG likely wins and it's size Lw Duran p4p would box the head off GGG if he were similar sized. I still think he could push GGG hard even with the discrepancy
Duran was competitive vs Hagler and beat Barkley who's bigger than Canelo, Golovkin. Duran would be competitive vs either Golovkin or Canelo. I think you're forgetting a past it Cotto went 12 rounds with Canelo and gave him a competitive fight at Middleweight.
Old Duran going life and death with the one-dimensional Barkley doesn't mean he'd be much of a force at MW for as great a pugilist as he is. Hagler underperformed against Duran, he was tentative and gave Duran too much respect. He was also concerned about his gas tank over the distance so went into auto-pilot for a few rounds and let Duran get into a rhythm. There's a reason that Hagler's Duran win isn't held in high regard and that's because he fought the wrong fight, struggled to adjust, and never imposed himself on a natural LW. I can't see Canelo or GGG having similar problems. GGG rarely respected his opponents, plenty of whom were better MWs than Duran. I'm not saying Duran gets stopped by either but he would get beaten up. And Canelo dominated Cotto btw. That wasn't a close fight.
