Aramintai

Of course he'd stab himself, it's easier to stab yourself than slowly die of illness in some jail cell for murdering a noble woman out of the blue.

Watch episode 2 again, it's explained in detail.
Mama Jin agreed to save Bu-Yeon's body, but that required to kill off Bu-Yeon's soul, which was clinging to Naksu's and blocking her energy. Master Lee used Naksu's energy and some sort of spell to transform Bu-Yeon's body to look like the sole soul left in it - Naksu's. It was done to reverse and stop petrification process - a body and soul must be identical so as to not leak energy and eventually run wild. But as a side effect Naksu lost her memories. But master Lee said that eventually they will return along with the body's powers. Now it seems that when Naksu remembers herself the body will start rejecting her again and she'll again become a soul shifter(?).

Even if it was serious and Yul had the authority of a judge/jury and the executioner - killing her without investigation and trial is just plain murder. He's clearly in the wrong, but too arrogant to consider other people's opinions or feelings on the matter. He's just planning on covering his crime by his swift approaching death.
I hate secretive, unbending people like him irl, never know what they gonna do behind your back for their principles' sake.

She stood before the different tree remembering her past, it was all imagination.

Let's just agree to disagree on what a really good person is. And fyi, the world is not black and white, nobody is always good.

Two different trees. Wait for future episodes - they will go to the real one.

That's what I kept saying since part 1 - he's a hopeless lawful stupid and prefers to keep everything to himself. Which does make him a bad, arrogant and unreliable friend. He should stand by his friends no matter what and try helping them, instead of doing nothing or trying to literally kill the problem.
And btw, Naksu is also his friend.

How killing Naksu is selfless? She's not doing anything. Whom it's gonna benefit?
Certainly not his friends. If he's gonna try doing it, Uk will not thank him, if that's what Yul's hoping for, he's gonna kill him himself.
Yul just continues to be an idiot with no real plan. He sat doing nothing in part 1 and got guilty for letting her go berserk at the end of it. Now he just decided to kill her as the easiest option. Instead of, you know, talking this through with his friends like a normal friend would, maybe listening to other opinions, finding better solutions.

He's on the side of lawful stupid. You don't hide secrets from your friends, especially not this kind. And especially you're not planning on killing your friend, if you're a real friend.
Yul needs to get his head out of his ass, his thoughts are too deep in it.

We gotta wait and see how they gonna resolve it. Something's gonna happen, master Lee wouldn't throw baseless shit into the fan just like that. Hopefully, this time it won't end badly.

Yea, I'm very pissed off that so many people know about Naksu now and not a single bastard deigns to tell Uk about it. The guy is suffering so much, it's hard to watch. Yul keeping silence is especially treacherous.

Master Lee reached Hwansu level of magic - at that level a mage cannot run wild. Uk is also at at that level or beyond, but that's because of the ice stone iniside him.
And the Queen sits on constant drugs Sejukwon keeps providing her.

Perhaps. But it's still better than being ejected, like some guys were panicking here after watching the early mistranslated version. Nothing new, gotta find another, more permanent solution to keep her alive then.

Looks like that Heo girl and her inseparable appendix are gonna dial biatch mode to 11 next week. She'll not only read a private letter written by Yul, but still gonna annoy FL with her dumb bouts of hopeless jealousy. I was patiently waiting through the whole part 1 for her character to make a justifiable sense of existing in this drama. But it seems she's just there to annoy everyone with her third wheeling. Move on already, b*tch!
Waste of scenes, if you ask me.

Yul is an idiot and a very bad friend. If only Uk knew what he was planning on doing.
This SML is not breaking any typical saeguk SML tropes, but his "noble sacrifice" here has got to be the dumbest and most unrequired I've ever seen. If he dies, it's because stupidity also must be punished.

So what do you, guys, think's gonna happen to Naksu when the body starts rejecting her soul? (*Rejecting*, mind, not *ejecting*, as some people mistakenly mistranslated earlier).

In the preview it looked like blue marks will return to her eyes, so will she become a soul shifter again and start petrifying, or what? To transform her body master Lee used both his spell and all of Naksu's energy, will the process simply revert back just like that? How does that even work, with what logic? Master Lee said it himself that soul shifters petrify because the body doesn't resemble the soul. And it was backed by part 1 explanation that this mismatch causes energy leakage.
Now he means what? That even memories can cause a relapse? That's some new shit the writers seem to be inventing on the fly now. I think they had to invent something to threaten her existence again and got everything so confusing and contradicting that it all starts looking very contrived. Same goes for how genetics are actually passed on - they put Bu-Yeon's body as hard evidence in favor of the body, but still keep other characters saying it's the soul. Pick one thing, you confused writers!

How is that a good ending? She's gonna become a bird and flies away?

I have trust issue with these particular writers. Can't trust Hong sisters with a happy ending.

Yea, so many people know about Naksu now and nobody cares to tell Uk about it.
Mama Jin, Jin Cho-Yeon, master Lee, Yul - selfish bastards, all of them.

Not you, specifically, but there are some bloodthirsty commenters here who advocated So-i should get a painful death.