An assassin.
murder.
Morbid
Yes it is considered suicide and it would be murder or assisted suicide, depending on how you look at it.
Your local police department.
The person is a blackmailer.
A knowledgeable person.
There is no minimum, and there is no maximum. The prosecution could call no witnesses or a hundred witnesses. A conviction of murder is based on the totality of the facts surrounding the person's guilt, not the number of witnesses called to testify.
A person?
a person
You call that person a scholar.
No, a Priest can not tell anything to anybody.
You would call them someone who peruses.