"You're a louse!" "I?" (Personal Pronouns vs Objective Personal Pronouns)

While reading Atlas Shrugged I've come across a number of conversations similar to the following:

Person 1: "You're a louse!"

Person 2: "I?"

This sounds very odd to me, but I can see how 'I?' could be short for 'I am?'.

I've looked around trying to discover if this is grammatically correct and I've learned about personal pronouns (I, you, he, she, it, we, they) and objective personal pronouns (me, you, him, her, it, us, them).

Personally, I think the conversation sounds much better like this:

Person 1: "You're a louse!"

Person 2: "Me?"

...with the reasoning that this:

Person 1: "He's a louse!"

Person 2: "Him?"

sounds much better than this:

Person 1: "He's a louse!"

Person 2: "He?"

Therefore I think the correct grammar in this scenario is the objective personal pronoun (me) rather than the personal pronoun (I). I'm not sure, though.

Can anyone give me some concrete rules about this scenario?