In the official translation the eldest brother Cheol sounds even more benevolent and tender to the 5th brother Ilchu. The way he guides him implying he doesn't even need permission, they way he speaks about Moryeon in a non malicious way but simply keeping an eye on him (and Seobun at that) I'm too hyper fixated in the brothers dynamic because reading between the lines and art and paneling it really seems like none of the brothers killed the second one. Now we know Yeju has knowledge due to being from the modern world, so whoever knows about the true poison could be another modern person
In the official translation the eldest brother Cheol sounds even more benevolent and tender to the 5th brother Ilchu. The way he guides him implying he doesn't even need permission, they way he speaks about Moryeon in a non malicious way but simply keeping an eye on him (and Seobun at that) I'm too hyper fixated in the brothers dynamic because reading between the lines and art and paneling it really seems like none of the brothers killed the second one. Now we know Yeju has knowledge due to being from the modern world, so whoever knows about the true poison could be another modern person