Something intentional was done on purpose. If a crime was intentional, it was no accident.
If you bump into your little brother in the hallway because you weren't paying attention, it's unintentional. But if wait for him to walk by and then stick out your foot to trip him, it's intentional. And not very nice. Usually we let people off the hook if they do something bad accidentally. But if your evil doings are premeditated, consciously planned, or otherwise intentional, no one will take much pity on you.