Morally, that is up for debate. But legally, it is not.
a witness
In the US, the only time you can say a person is guilty of a crime is after they have been convicted of the crime. Until a person is convicted or admits guilt in a court of law, they are charged with the crime or suspected of the crime.
Not unless a court/jury finds that person GUILTY of the crime. Being charged is not the same as being convicted. Without being convicted, the person's record is clean. Meanwhile, if charged.... Get a lawyer! Say nothing until having talked with the lawyer.
No, the witness cannot use 'John Doe' when interrogators are asking a witness to a crime for the full name. The police do their job well and they are there to listen to the witnesses side of the story whether they know the name of the guilty person or not.
the defendant is the person who is found guilty or not guilty of the crime commited.
Said person would be an "accessory before the fact" and would be guilty of what ever the perpetrator is guilty of
a witness
A guilty party is the person (or occasionally an animal) who actually committed the misdeed or the crime.
he now what's going on but he can tell nothing
self incrimination
No
In almost all cases, no. You would not be guilty, but if you saw what happened, you are a witness.Another View: The above is true only if you were truly an un-involved witness who just happened to be at the scene when the crime went down.HOWEVER - if you were there accompanying the person or persons who committed the offense, YES - you could be charged as an accessory.