I blogged below about what happens when the same conduct is both criminally and civilly actionable. But, my student also asked, how does the legal system decide that some conduct is a crime, some conduct is civilly actionable, and some is both?
1. Civil Liability Is for Compensating the Injured, Criminal Liability Is for Punishing the Morally Culpable. This is an oversimplification, for reasons I'll mention below, but it's a good place to start.
Thus, consider attempted murder -- someone shoots at me but his gun jams and, for good measure, I don't even learn until later that he tried to kill me. Attempted murder is a very serious crime, because people who try to kill others but fail are usually thought to be nearly as culpable as those who try to kill and succeed. (Some people view the two as equally culpable, but the legal system generally doesn't take that view.) But it's not a tort, because there's no injured party. Same for drunk driving, which can be punished even before the drunk driver causes an accident.
In some situations, for instance tries to knife at me and misses, he might be guilty of some tort that compensates me for the fear caused by the attack (e.g., assault or infliction of emotional distress). But in many cases, there's no civil liability at all for an attempt, because there's no injury to be compensated.
On the other hand, say you do blasting on your property, you're as careful as possible (and are therefore not negligent), but my neighboring property is still damaged. You're civilly liable to me, because this is a strict liability tort. But you're not criminally liable, because we generally don't punish people unless they're morally culpable, which usually (though not always) requires at least gross negligence.
2. Of course, as I said, this is an oversimplification. To begin with, civil liability sometimes involves punishment for the morally culpable, for instance in the typical punitive damages case, or when a tort requires some highly culpable mental state, such as knowledge that a harm would likely be inflicted. And the criminal law sometimes imposes strict liability, though that's pretty rare.
3. And beyond this, there are also lots of other practical factors that counsel in favor or against criminal or civil liability in some situations. Thus, for instance, most states have abandoned criminal libel -- either formally or in practice -- because of a concern that criminal libel would unduly deter constitutionally valuable true statements and opinions (and not just false statements of fact). Libel may still be morally culpable in many situations, and may lead to punitive damages liability as a result, but the criminal law generally doesn't punish it.
Likewise, the tort of intentional infliction of emotional distress is usually limited to behavior that a jury finds to be outrageous, which is by definition quite morally culpable. But there is no general crime of intentional infliction of emotional distress, partly because the definition of the offense is probably too vague for the criminal law.
Conversely, some relatively low-level harmful behavior is not highly morally culpable, but civil liability probably won't do a good job of stopping it -- perhaps because the behavior is committed by people who don't have enough money to make a lawsuit worth a plaintiff's time, or because the harm is spread over many people and none of the potential plaintiffs can sensibly afford to sue. Consider, for instance, graffiti, being drunk and disorderly when the disorder consists just of shouting, and the like.
In such situations, the behavior might be made criminal chiefly because only the criminal system can do much about it. Civil liability may theoretically be available in those cases, but practically it isn't used.
4. Finally, let's step back a bit: This is a good example of a common legal phenomenon -- there is an important theoretical distinction between two kinds of legal approaches (here, compensation for the injured vs. punishment for the culpable), but the distinction explains only part of the story. It's important both to understand the big theoretical distinction, and understand that it's not the only principle in play.