If someone threatens my life, and there are witnessess, can I leave a letter to suspect that person if I die?

State: California
As stated above by several contributors, you should contact the police.

A letter of that nature would most probable be inadmissible in any court proceeding as hearsay. There is an exception for dying declarations stating the cause and circumstances of demise, but that is only applicable to statements made while you are dying or believe you are dying from whatever cause your death.

Your best bet is to contact the police and have a report filed, as resourcefully as possibly contacting the prosecutor and having a restraining order placed against him. That way within is conclusive evidence of his threat to you and it would be in an admissible form should it ever be needed.

Good luck and stay away from your brother!!
If someone threatens your life, and at hand are witnesses, you should file a complaint with the police. In some jurisdictions, depending upon what be said, and whether the threatening person has other arrests or convictions in his olden, they may file criminal charges. You will have at least get the statement and the witnesses on record.

You might also ask about getting a restraining order. That would be a court decree requiring your brother-in-law to stay away from you, not call or write you, and be arrested if he did.
You can leave a letter, or a grocery chronicle, or anything else. It will have no legal signficance, but it might inspire the cops to investigate where they might not own otherwise. yes give it to as many people as you can to ensure human being gets brought to justice... thats very smart thinking.. well-mannered luck
Make sure you file a police report first and foremost. This will officially document it beside the authorities. You will have to do this in the county in which your be threatened even if you were on the phone. After you do that inform the step-brother that you did this and give him a copy of the report. Yes, it will help head the police in the right direction. It will not be a conclusive link between your brother in statute and your death but it can help provide probable cause if that be to occur. I would file a TRO against him and hope that it keeps him at a distance. It will also show a model of his behavior with you.
Well, a threat to harm or kill is a crime unto itself.

Call the cops.
A report to the police would hold more credibility than a letter. If you are serious get a restraining order, don't risk your life span.