What's a sage king?? Why is Gwanghae being hated? Apart from the murders of his family members which he did not actually order, Gwanghae is doing his best for his people. So why is he an 'evil' king? Is there any king who didn't murder for the throne and 'the greater cause'? Isn't a king's priority is protecting his people and granting them a better life?? That's what Gwanghae is doing. So is he a sage king??

According to my understanding (based on my reading of Wikipedia and my, admittedly, very limited knowledge of Korean history of that period), Gwanghae simply didn't have the support of the majority of court nobles (and bureaucrats). I don't why know exactly, but they made a big deal out of him not being (1) the eldest son of the late king, and (2) the son of the queen. (But these seem to be mere excuses, and that the real reason behind their rejection of Gwanghae had something to do with factional politics.) Since they eventually succeeded in dethroning Gwanghae, and bearing in mind that history is written by winners, it's not surprising that they would make him look bad.

I honestly don't know if he's a "sage king" or not, but he's clearly fitter to rule than his successor, the weakling Injo!
I understand the reason why he was not agreed on by the politicians. But what I don't understand is why his people and many of the drama audience are hating his guts. Honestly, compared to many other kings -I watch a lot of historical-, Gwanghae is not as merciless as most of them. On the contrary, he's pretty caring about his people. I'm not justifying any of his murders -whether he directly ordered them or not-. But what I say is that his good deeds are way more than his evil ones. And from what we watched so far, Jeongmyung's plans are all aimed for revenge and nothing relates to people. The reason I'm liking Joo Won so much is that though he hates Gwanghae personally, he knows he's the right king for the era. 
Really? I don't even realize that (many of) the audience hates him. I know I don't. Are you talking about the Korean or international audience?

I don't know, maybe some viewers choose to apply a simplistic approach to judging his character. (As in, they hate Gwanghae because he's the villain of this drama, and because he had the heroine's brother killed, period.)
The more I watch of Gwanghae, the more I grow fond of him. It is devastating to see him being lead my the court lady, but that is the kind of support he has wanted to keep by his side. The woman is smart - that we have to admit. In the last episode, the map scene where he talks with In Woo about wanting to keep Joseon separate from China is so beautiful yet heartbreaking. To see that a king has an unselfish plan is new to me. I like Gwanghae, and it'll be sad letting him go when In Jo takes over.
I kind of agree that most people would judge Gwanghae for the evil things he's done in this drama but I think people would fail to realize that the environment he was in made him that way. He rose to the throne through hard work and waited a long time for it too, but everyone was against him becoming King because of his illegitimate status. Gwanghae had all the qualifications to be a great King, but the oppositions and pressure he had gotten was what made him so psychotic about keeping his place on the throne. Just like many other Kings who reigned, Gwanghae was just a puppet for the factions who actually controlled the nation, but even then, he managed to accomplish many things for the people. Every time I watch this I feel so conflicted because I pity Gwanghae and want him to do well but I find myself rooting for the Princess too :(. 
I just finished episode 5 so I can't say much - But I personally like Gwanghae and think he is a good ruler - very misjudged by his so called "family", Since he didn't order the killings of the princess or the young prince - that means he's kind. It's the Court Lady that is PURE EVIL - I honestly can't stand her guts and I would love nothing more than to see her dead. lol - I'm disliking Jungmyung b/c she is believing what everyone is saying instead of holding onto her trust in her brother.... I get that it's a power struggle but he has done nothing that should make her hate him - she has started mistrusting him and hating him b/c of HEARSAY and I hate that kind of grown hatred and mistrust the most. I can tell I'm going to have a hard time watching this b/c of her and the dispicable Queen & the awful evil as can be Court Maid.... I feel the urge to scream inside! My chest feels stuffy with all this political strife! (and that's coming from someone who just finished a 50 episode 100% Political based historical drama prior to moving onto this one - but that drama was much easier to watch - this one has me mad as so many people when all I can do is pity Gwanghae and cheer him on!)... Does anyone have a Historical account of Princess Jungmyung? b/c as far as I know there is nothing about her... is this all made up? (her parts/role I mean)... UGH.... Just leave poor Gwanghae ALONE! I mean sure I have conflicting feelings about Gwanghae at times too but not enough to make me hate him - I like the dude!