Wait... so the caster carry's around a bag of rats. Because that seems like something you could buy in a store and rats - that can chew through wood and cement - totally wouldn't escape.
Then you cast hex on a rat. So hex is up at the start of the day. Then you rest because the RAW says you "regain all expended spell slots".
Okay, with you so far.
Then, you kill the rat at the start of combat, releasing the hex to target another creature.
But... that requires an action rather and a bonus action than a bonus action. So you're up a spell slot but down a round of combat. Oh, and you need to figure out which rat in the bag of rats you cursed. Did you take it out and put it on a leash or something?
And you could fail to kill that rat with a poor roll.
What happens if the rat escapes? There goes your hex.
It's such a ridiculous tactic that should in no way work.