A policeman said to his colleague "That guy has popped people for a lot less than being undercover cops"
I know pop means shoot but what does the above sentence mean?
"That guy (man) has popped (shot/killed) people for far less harmful/far more innocent things than being undercover cops (police officers who entered his organization in order to spy on him and get him arrested)".
So, if he has killed people for something like, say, tryin to rob his wallet, discovering an undercover police officer in his organization deserves no less, and it's no surprise at all that he should kill him as well.
Please be aware that I'm neither a native English speaker nor a teacher.