Well, that's because Jing is the emperor. People believe that the emperor is the son of heaven, they have the supreme power to rule the whole nation, Another wards, his words are absolute. (<-- doesn't mean that he's not cruel but I could understand how he came to be like that).
With his beautiful looks, great martial artist skills, and powerful position, no wonder nobody ever disobey him (other than Lu Cang and his uncle), Even if Jing is wrong, I'm sure he learned to justify his action with his crown.
However it does seems that he is a great leader.... but a shitty lover.
well, aside from what bakamono mentioned, jing's arrogance actually stemmed from that mindset. like, it's also necessary to keep up appearances. the authority, knowing that is was yours. everysince he was born, educators have been driving it up his head. coupled with how very few were educated and monarchs are the most educated ones so his confidence is unparalleled, and with this knowledge he fully believes that he has every right to do whatever he wanted. we may not fully understand is as modern people of democracy but that's how they work in the ancient times, moreever it's a place with old monarchy too.
i have also finished the novel. when jing admitted his feelings he also suffered and did what should not and never be done by monarch of their time. that part is so sweet. i think part of the reason of his cruelty is in a way he is denying his feelings for someone that he is not really allowed to have a relationship with as 'the son of the heavens'. but he broke taboo and that strong love he feels is enough to make me forgive him.
Why Jing is such a cruel man? He may be sexy but he still is a bastard!! Read the novel to understand how terrible he is!! I feel so sorry for Lu Cang.......