Of course, I have to explain that why bad guys are bad. Imagine what will happen if you live in a community, where you cannot go to school without worrying being robbed, stabbed or shot by bad guys.
Also need to explain why there are bad guys. In my opinion, there are bad guys because they have nothing to do. For one, there is no income so they tends to do things illegal. Also, since bad guys are rarely educated, the extra time is not used to entertain himself in a better way.
That is the designer of this society should keep in mind, because there will always be someone has no income and extra time.
Finally, I have to explain that policeman and us are no different. Facing a threat, one has to determine the level of threat and only respond in a certain level. If two kids fight, Jimmy said, it is OK because kids keep fighting. And if there were a bad guy doing nothing wrong, even policeman cannot take him to custody. If someone quarrels with you, do not respond with fist. If your life is in danger, then you can do anything to protect yourself, even with guns. Thus policeman is doing the same thing.
Think it in a broader scope, if a country is threatened, it has to determine the level first.
Jimmy again asked if bad guys set a fire in our house, what shall we do? That I don't know how to answer. Just told him a different person may treat strangers as unknown threat, thus may shoot without hesitation. And he said he will not enter another person's house without being invited.
So much endeavors for a big topic!