Above all this isn't a place to call out bad behaviour to begin with.
The meaning of "to begin with" in this usage is "before, or regardless of, any other consideration". People often use "in the first place" as an alternate expression meaning the same thing in this usage. It is basically saying that it is inappropriate to call out bad behavior here, so any finer points of discussion are irrelevant.
I looked for an official citation but the dictionary definitions for this case are pretty bad. The thesaurus lists "in the first place" as a synonym, and that was only marginally better. TheFreeDictionary.com cites The American Heritage Dictionary of Idioms. They give this similar example: "He could have bought a new one in the first place." The associated definition is From the beginning, at the outset, before anything else.
[What] I'm doing is explaining bad behaviour to begin with.
Without some context, this is a little ambiguous. However, it sounds like "to begin with" here means "as the first step". The meaning of the sentence would be similar to, "First I'll explain what bad behavior is, and then we can talk about it in the context of this situation."
This would be related to the definition as firstly: "There are many reasons why I don't like her – to begin with, she doesn't tell the truth."
"To begin with" in this usage is sometimes not explicitly intending as the first of multiple things, but a related meaning of just establishing a starting point or baseline. The meaning in this sentence would be something like "[What] I'm doing is explaining bad behaviour in general".
An alternate interpretation of the definition would be that this case is also similar to the first. The meaning "at the outset" could be applied to the process of the discussion, itself, rather than to the situation being described in the content of the sentence.