In the humorous exchange in the Avengers I， Bruce said:
“I don’t think we should be focusing on Loki,” Bruce said. “That guy’s brain is a bag full of cats. You can smell the crazy on him.”
Thor took a step toward Bruce. “Have a care how you speak,” he warned. “Loki is beyond reason, but he is of Asgard . . . and he is my brother.”
“He killed eighty people in two days,” Natasha pointed out.
Acknowledging this, Thor backed down a little. “He’s adopted.”
I could not locate the exact meaning/implication of "cats" here. So far I have three guesses:
1) There is a phrase "let the cats out of the bag", meaning secrets leaked; so does this mean "Loki's head is full of (dark) secrets?
2) According to Collings dictionary: "a bag of cats" is an Irish informal way of speaking someone "bad-tempered"; so does this mean "Loki is a bad-tempered person? (which I personally don't think it fits the context and the usage of Irish English does not make sense to me in an American movie.)
3) In my culture, cat has some connotation of "smart\clever", which leads to "sly"; so does this sentence mean "Loki's brain is very smart and he is sly?"
Anyone could help me with the "cats" here please? Thanks a lot.