fear of rewarding (or not correcting) "bad" behavior

I understand this fear, but I do not think it is an actual danger. Here's how I see it: When a child acts out (meaning that instead of feeling or directly expressing their feelings they act them out kind of sideways, in hurtful or unworkable ways, that always means that they are feeling disconnected and/or the feelings are too much for them to handle right then. Either way, they are not in any position to take in our information about appropriateness or proper behavior. If they could take that in, they wouldn't need it! Rather, they are overloaded with feelings, including the feelings that arise from disconnection. Therefore, the number one response is always to reconnect. Part of that reconnection is by reflecting or mirroring the feelings that the child is feeling (knowing that we are usually only guessing here, but we can make some good guesses). We need to recognize that it's too vulnerable, or just plain impossible, for a child to express those feelings any more directly than they are doing it. So we help them express it (like I did with that boy who said "I hate you." and I responded, "I had a good time too and I'll miss you till I see you again." The result of this empathy and reconnection is that the child "returns to himself," and then is naturally cooperative and thoughtful. This is people's natural state. At that point, when they are back to themselves, you can tell them about appropriate behavior, but it won't be your words that make it work--it will be the reconnection, the stabilization and regulation of feelings, and the resulting positive behaviors.
If you are determined to give that lesson about inappropriate behavior, you can do it in a way that maintains and even builds connection, instead of ways that disconnect (I guarantee that the use of the word "inappropriate" is a disconnector for most kids). An example is scooping him up with a big smile and saying, "Oh no you don't!" No language like that around here, let's try that again." Your smile and bright tone keeps him connected, not conveying he is in trouble, and yet you convey your true feelings about the situation. Harshness is not necessary or helpful. try it!