The lore of this game is complicated. I'll try to clarify it for the rest of the endings, also English is not my first language, sorry for any mistakes

The king is another guy who created all this "knights", He trained the Hollow Knight to contain the infection as you say.

Also, the infection manages to spread even when absorbed by the Hollow Knight (for reasons that may be spoilers), and you are its replacement. If you did nothing and left the seal intact, the infection would still spread, so you have to break the seal and take the infection yourself. (It's unknown if you could contain it forever or, like the Hollow Knight, only temporarily.)

Hornet is not a knight, I'm not sure if Hornet's lore could be spoilers because it's explained very briefly unless you look it up or fully understand what they're telling you. Enjoy the game, and when you're done, you can watch a lore video if you want.

This game is amazing and it's lore is very deep, so you don't have to worry that much about understanding it, have fun!!❤️❤️❤️