So, my friend has this theory: he thinks that the Lord Ruler, during his fight with Vin, used a Duralumin burst with Atium that he had compounded, letting him see several years into the future. He decided that Vin could handle Ruin better than he, and, since he was sick of living (I mean, who wants to live for a thousand years where your noblemen hate and fear you and your vassals hate you), he let Vin win.
Tell me what you think.