You say it's bad roleplay to break imperial law, and you're right. But the bad roleplay is not because it's just "breaking imperial law" it's that the character wouldn't *want* to break imperial law. That's the player, ooc, wanting to break it because they want to pk someone, or get some equ, or whatever. The *character* wouldn't want to be acting that way, regardless the other things. What's important to *the character* is upholding order and darkness, visibly demonstrated by his joining the empire and having the bloodoath magically enhance/bind him to those beliefs.
Think of it this way. In the real world, you may see someone with something you want. But you don't kill them for it, do you? Because to you, that would be "wrong". You don't want to do something "wrong". The same thing applies to the empire character. Attacking someone in a protected town (or whatever imperial law you want to discuss) would be "wrong" and he wouldn't want to do that.
So doing it means you aren't being that character, you're being you the player.