The father never tried to assassinate an emperor as far as we've seen so far. He succeeded in assassinating a viceroy. If anyone killed the old emperor it was probably the current one, you know how it is. "Court politics." The attempted assassination this time was a misunderstanding, which has been cleared up and even rewarded if anything because the emperor wants to train this "boy" up himself. I could be wrong and maybe the father did actually kill the old emperor, and this one just seized the opportunity to rise to power. The old emperor kind of had a hand in the wife's death and all, being a corrupt twat and employing corrupt twats. But I figure it's the first way around rather than the latter.