> I hope we still live in a world where "have fallback measures in case other people are going to murder you" doesn't need to be at the top of our contingency planning. Obviously it was here, and that's tragic, but I think and hope that most of us shouldn't need to learn to anticipate this as a likely, or even plausible, response.
Sure, if you only consider murder, then it's unlikely (in America at least). But this same pattern happens for lesser things. Someone hits your car, and says "can we just settle this privately, I don't want my premium to go up", and you agree out of kindness, and a few days later you get a letter saying that they're making a claim against your insurance. You've missed your chance to get yourself on record as the victim.
In this case, or similar cases, the guy might have done other things like accuse him of blackmail, or accuse him of being involved in the crime etc.
Notwithstanding the risk of being compelled to testify, I would still take the route I mentioned above (report it but refuse to cooperate any further). I'd be surprised if the prosecutor decided to press ahead and grant you immunity to make you testify if you're not willing. You could retract your statement, which would signify that they cannot predict what you'll say at trial. Much more likely they'd just drop the case if the victim is unwilling...but I'm not a lawyer, I could be wrong.