Please explain how a perpetrator's state of mind is relevant to guilt in committing a crime

In most legal systems, it is required to demonstrate that the perpetrator of a crime had intent to harm, also known by the legal term "animus nocendi"(from the Latin animus "mind" and noceo "to harm"). Put plainly, this means having foreknowledge of the illegality of his act and the repercussions of the crime. Intention is important in establishing guilt for a specific crime. A good example is a car accident fatality: the driver responsible would be guilty of causing the accidental death of the victim (manslaughter), but not guilty of murder -- unless it could be established that the driver at fault intended to kill the victim.