
That's the same for just about everyone in the palace, from the Emperor to the consorts, etc. They all choose someone to support. He didn't father the child, and that's what he was put to death for. That's why I sympathize. The real culprit is the Emperor: he is a fool, and he doesn't keep his word (to the first wife). Because of his fickle emotions, the women have to live in an uproar. But yeah, he's not fully innocent, but he didn't do the crime he was killed for, and he was another dupe for a plotting woman.

I agree and I do think his ending was sad but if you participate in the harem battle then you must be prepared to lose your life. It's a ruthless battle, he picked a side and even if it was innocent, he still helped her(which would indirectly hurt a lot of other women in the harem). Him losing his life is because the side he stood on lost.

I guess I see it as inevitable. He was her childhood friend, and he was a prince--there was for him not really an option but to help her. Helping her seem to be empress by sign and the child blessed by sign are pretty mild compared to real harem/palace maneuvering (ie, lying to get someone killed, or poisoning, or other things that get people killed). The Empress was sidelined by the emperor, not by pink-hair. Pink hair was surviving how she knew (tears, manipulation). Truth is, the real enemy was and is the emperor, and he's the one who should die for allowing the cruel harem situation and breaking his promise to the queen. He's the lying, cheating bastard who should die. And he's only a cheating bastard cause of the promise and sidelining the empress. The fact is, an emperor could have many wives and concubines...and the empress knew that. It was common in many cultures for rulers to have multiple women. If you want a man to be faithful to only you, don't marry the Chinese emperor. :-/ .

I think they just ment that by aiding Liu he automatically becomes an accomplice. He knew what would happen and did it anyway. But if you look at the reason he did it, it's not selfish like everyone else's reasons are. They looked out for themselves, he looked out for the woman he loved.
Which is also why I pity the original host of the MC's body. She, out of everyone, truely loved the Emperor enough to sacrifice herself multiple times (and not for the sole reason of advancing her political status) but as soon as an new girl came along the Emperor tossed her aside like a broken toy and chose Liu over her repeatedly.

Exactly like LalondeBii say an accomplice. If you put together the two pages it get a little easier to understand the original timeline.
http://www.mangago.zone/read-manga/cheating_men_must_die/hs/mpn_chapter-2064359/pg-3/
http://www.mangago.zone/read-manga/cheating_men_must_die/hs/mpn_chapter-2069837/pg-2/

And if we read this latest chapter, she (pink hair) was reincarnated after herself being bullied and abandoned and dying in the cold palace. So, she's like a revenge MC coming back to get her own. That makes her actually more sympathetic. How is the new mc any different than the pink-hair (coming back to rise up and kill her rivals.)

But it exactly like what the empress said. She knew how the harem is, but still in her second life choose to enter, and schemes to to hurt the empress, and don’t forget the dead child of the other concubine. Is the empress the one who hurt her in the past life? It was the emperor that let all that happened. But what happens to the emperor, the main culprit of her demise in the past? She suck up to him, loves him, bear his child, hurt others for him. She was pitiful in the past life, but this life she choose it on her own violation. She knew the harem game, still willingly participated in it. Her ending and the emperor’s are befitting, only, the emperor should have suffers twice as hers. Having many wives doesn’t mean it’s fine to have some suffers. If he can’t handle his harem, he should not have one to begin with.

The emperoris shit, but we have lots of these reincarnation ones where the woman tries to get revenge and the guy loves them this time when he didn't love them the time before. She's like an example of that. In her case, she didn't want revenge. She wanted the Emperor. And if the empress had reincarnated, herself, who is to say she would not still want the emperor and fight for him by killing whoever got in the way? The previous empress should have killed the emperor, not herself. the previous pinkhead should have killed the emperor (and does now), but she wanted his love and she got it (for a while). the only reason the emperor got punished is cause this is not a woman who loved him--she's an avenger/assassin. That is why she can be cold and calculating about harming him. The other two women--pinkhead and empress--actually loved him and wanted to be loved. So, it makes sense that in reincarnating, pinkhead figures out how to maneuver for his heart.
Also, she may not have had the choice to not be in the harem.....she may have reincarnated already in the harem (like in so many of these transmigration or reincarnation stories ,they don't go back to being babies or kids all the time.)

It stated in one of the earlier chapters that Liu (pinkette) reincarnated from a past life where she read the book. It's also stated that she was willing to murder to do so, heavily implying that she has already killed people.
Liu didn't really love the emperor, she just wants to be empress. She was good at acting in love, but can you honestly say what she did had any root in it? Look at the original host of the MC's body, she got to her position because she was willing to suffer and die for the emperor whereas Liu was only willing to torment/kill others for the emperor (for the main purpose of ruling next to him- I think it was even said she grew up poor-) when it really wasn't needed. All the people she hurt loved him.
As for the difference between the actual MC's actions and Liu's? Motive and social (or mythical) standing/duty. The MC wanted to enact revenge on behalf of the woman who was so wronged she killed herself (which, yeah, not good. But it's still better than Liu's reason) and Liu had a get rich quick scheme that didn't care who was in her way. Add in the fact that the MC is basically a divine being acting as both judge and executioner for other's sins while Liu is a reincarnated girl who wanted to live in the lap of luxury (even though the 9th Prince genuinely loved her- and we're made to believe she loved him back when we see her reaction to his death- and could have given her a comfortable life) as empress.

I can't remember the name, but it was about a goddess (cough- reincarnated from another world where this was a game- cough) loosing power and having to make people fall in love with her to get it back. The more difficult it is to get a person to fall in love, the bigger the power up. There was a 'mission' with a harem that was a lot like the one here. It's been a while since I've read it actually, about a year or so.
Kinda feel bad for the 9th Prince. He just wanted Liu to live a good life, even if it ment he couldn't have her the way he wanted.