Why is Muzan genuinely so stupid

[VigilantIcy]

This question goes through my mind over and over again Muzan had the potential for something very great I won't be discussing manga demon slayer since this is an anime discussion topic but what rly irks me is the wasted potential on certain demons ill start off with one of my favorites susamaru I think this character had so much potential I also really like her design giving off that asura vibe but Muzans curse was able to get to her because of the spell tamayo placed on her Muzans curse is one of these dumbest things in the entire show people know his name by now ubiyashiki, all the hashira, past hashiras including urokodaki so why does this curse still need to exist susamaru had the fight in her hands very easily an injured tanjiro from yahaba she outmatched yushiro by finding his weakness with dust that nullified his invisibility and as tamayo stated when she was going to go all out she had nezuko defeated all Muzan had to do was not have the curse I'm fine with her losing the fight and running off because of the rising sun but killing her when she had the fight in her hands was so dumb 

[Moxie]

Because he is not Main character 

[Alli]

Dude was stupid and delusional at the same time. really thought he could beat all demon slayers and conquer the sun.