Criminals can already do bad things.
Since they're willing to break laws, they already have lots of options available that provide better privacy than Tor provides.
They can steal cell phones, use them, and throw them in a ditch;
they can crack into computers in Korea or Brazil and use them to launch abusive activities;
they can use spyware, viruses, and other techniques to take control of literally millions of Windows machines around the world.
Tor aims to provide protection for ordinary people who want to follow the law.
Only criminals have privacy right now, and we need to fix that.
Some advocates of anonymity explain that it's just a tradeoff — accepting the bad uses for the good ones — but there's more to it than that.
Criminals and other bad people have the motivation to learn how to get good anonymity, and many have the motivation to pay well to achieve it.
Being able to steal and reuse the identities of innocent victims (identity theft) makes it even easier.
Normal people, on the other hand, don't have the time or money to spend figuring out how to get privacy online.
This is the worst of all possible worlds.
So yes, criminals can use Tor, but they already have better options, and it seems unlikely that taking Tor away from the world will stop them from doing their bad things.
At the same time, Tor and other privacy measures can fight identity theft, physical crimes like stalking, and so on.