Ignoring what he did to Excelia, Yuukiaru was really dealt a bad hand. He discovers his family line to be liars just like the conmen who mocked his now-false belief ("family heirloom" does not morph into "direct lineage" via oral tradition without someone fibbing), so the only soul he could cling to was the very sword of the Hero who gave him a chance. That's years of his life crashing down around him.
If he had dug into more of the records, he might've actually found proof from later in the Hero's line, but he goofed it.