خبث is the plural of خبيث.
خُبُث is the plural of خبيث, you have to also note that خُبُث is not a plural for خبيث in classical dictionaries, only in modern ones, and even then it’s listed last. The most common خُبثاء وأخباث وخبيثون.
The Hadith says خُبْث, not خُبُث, which is the opposite of طيب. It is a the quality of being bad, harmful, or malevolent.
الخبائث is the plural of خبيثة when it refers to humans, but it doesn’t refer to humans here. Here it refers to the same thing as the aya does here:
وَيُحِلُّ لَهُمُ الطَّيِّبَاتِ وَيُحَرِّمُ عَلَيْهِمُ الْخَبَائِثَ - الأعراف
وَنَجَّيْنَاهُ مِنَ الْقَرْيَةِ الَّتِي كَانَت تَّعْمَلُ الْخَبَائِثَ ۗ - الأنبياء
In this case الخبائث is the plural of both خبيث as well as خبيثة since it refers to غير العاقل.
It’s clear from context, you say that when going to the toilet, you shouldn’t encounter any humans in the toilet, but you will encounter الخبائث. The reference is to whatever is coming out of you which is definitely not طيّب!
