A witness.
Doubting Thomas, from the disciple who would not believe that Jesus had risen from the dead until he saw Him.
A person who commits a crime is known as a criminal.
a witness
They are called the Coroner.
An eyewitness.
Criminal sociopath.
Such a person would be an accomplice or a partner in crime.
an accessory after the fact may be what you are looking for
Yes it is. In fact it's a crime even if he was coming out of your house.
greedy, unethical, with out shame or morals
eviedence
It depends on whether they think the person is part of the crime as well. If they do, then the person is called an accomplice... someone who helped the suspect commit the crime. If not, then the person is just an associate... someone who knows the suspect and might be able to identify where he or she is.