I know that you dont like it when ml's get clingy... but, he just saw his (what i assume) lover get blown up infront of him, he tried his best to save him but couldn't. Do you know the feeling of being unable to save a person that is dear to you? Doing everything you can, but they still end up dying?
Also, he was abandoned by mc, and I guess had to live the rest of his life without the mc, bro has magor trust issues and can't feel at ease.
Now, I'm not trying to be rude, but I'm just suggesting to look a little bit deeper, and try and piece things together.
Also, what makes this story a bit icky, is the translation, if we had a better quality of translation, we could actually understood what the hell is happening.
But yeah, you have the right to not like it, the stories characters etc.. I am not trying to convince you to like it, it's your choice.
personally i don't really like this current arc mainly bc of this X slime character. i dont like them at all actually i just think they're annoying and weird. most other people seem to want the backstories of mc and ml but i just want to see mc as more villains lol. i find that much more entertaining and i dont like when each version of ml gets so clingy.