Wait, when did this become about Jilius? I thought we were talking about Sorrow.
Anyway I really don't have a lot of mental energy right now, but I want to try and bring some things up. Sorry if it's kinda disjointed and confusing, it's hard to think straight lately. I do agree with a lot of what Vokadae and AlphaRed said, too.
As far as Sorrow goes, he was desperate and wasn't thinking rationally, and while, had he succeeded, he may have regretted his methods, I don't think what he did was "idiotic". He was spiteful and bitter over being branded as an evil thing, because no one wants to be deal with the emotion he represented. And he's a literal ball of sadness, so he would obsess on the negative feelings of that situation for however long he was holed away. Of course he would resent Lunatea and want to take out his feelings on it, wouldn't you?
Not to mention we have no idea what the circumstances of his/his kingdom's disappearance were. There's nothing at all saying that he didn't try to reason with The High Priestess, or anyone else in Lunatea before resorting to drastic measures. There is zero evidence either way of that, we just don't have the information to claim that as a fact. He could very well have tried for a long time. Or maybe he was too spiteful to ask these people who shunned him to take him back. I wouldn't say that would be out of character, either.
And regardless of any of that, he was bitter and miserable, and acting purely on negative emotions without any regard to consequences. If he wanted revenge on Lunatea, I don't think he would give a damn about whether they saw him as evil at that point, since they already did. If it's been ingrained in their religion for possibly thousands of years that this bell is evil, and they've all forgotten it was even another kingdom and not just an Evil Bell, I don't think they would be swayed that easily. It could even be a risk to try and convince them as Baguji, if he loses his respected status for spouting such "crazy ideas". It's far smarter to play into the world's already established, if unfounded, fears and legends to trick them into getting his kingdom revived, imo. Aside from that, self-destructive thinking like giving in and calling himself evil isn't exactly out of character for someone as depressed and unstable as him, if he was being genuine and not just tricking everyone.
And anyway in the end, he did not get killed? At least not really. The sorrow we met died, but in a way he seemed content with, and he was reincarnated. Happy end.
He was not "insane and needed to be put down". He was lashing out at Klonoa with who knows how many years of pent up negativity. When he was defeated he was burned out on those emotions, and again asked for help like he had been the entire game. He's emotionally volatile, not insane in the slightest.
I think if you had spent thousands of years living in misery that slowly bubbled up into spite and bitterness, you would lash out, too.
King of Sorrow is a good and relatable antagonist who made some questionable choices, but he is not an idiot or insane.
Sorry if this was really just, indecipherable, I'm not in a good place to write essays lately. I tried lol