So I’ve read that Kant was a classical liberal, but I can’t quite understand how that could work with his system of ethics. Wouldn’t his idea of the categorical imperative require the state, or whatever else protective agencies, to never kill, maim, harm, banish, imprison, etc.? So in that sense, wouldn’t property rights and individualism fly out of the window because Kantians would not protect their property from thieves and robbers because it’s considered immoral to kill, hurt, or to imprison?
Kantianism consists in self-legislating in accordance with practical reason, or equivalently acting as an autonomous agent. The reason aggression is immoral under Kantianism is that it leads to a contradiction in conception - it is a maxim that cannot be universalized without falling into logical contradictions. There’s nothing in the Kantian system prohibitive of self-defence. You need to understand what the categorical imperative is, and why it is immoral under the Kantian system to do any of those actions. I think Kant did not believe the moral principles he enunciated applied to the state, but I might be wrong. Either way, use of force in defence is not something that would lead to a contradiction in conception.
-Jon
I figured you’d be the one to answer the question. 
Thanks. Do you know of any good works about Kant that I can read or should I just read one of his books?
I would suggest just reading Kant himself. Groundwork of the Metaphysics of Morals is a superb work on ethics by Kant and not too difficult.
However, don’t confuse Kant’s ethics with Kant’s politics. He wasn’t very good (in my opinion) of applying his ethical theory to political theory and was also far too enamored with the State. He often drew parallels between moral law and civil law, in that both should be obeyed simply for the sake of the law itself, however, as we all know, civil law is hardly something to be held up and celebrated.
Just part of the problem I have with Kant’s ethics and much of deontology is that it treats ethics like law. It’s not. Granted, the term ‘natural law’ kind of confuses the issue, but traditional natural law ethics was Aristotelian/Thomist and didn’t suffer from this problem.
Christine Korsgaard is a top Kant scholar though. She has a book on Kantian ethics, not exegetical, called The Sources of Normativity.
For a good selection of Kant’s political writings, see the Cambridge edition Kant: Political Writings edited by Reiss.