I'm going to rat on someone who is planning a coup.
Are you sure you have the right person?
Yes. He planned a military coup and I'll be doing a service to my country by telling the police on him.
If you genuinely believe this person is planning or has planned a coup, then you definitely need to contact the authorities. This is a serious matter and can cause damage and injury to other people.
I think he's going to cause an uprising and the military is going to storm the capital.
You need to hurry up and call the police on him. That is a serious crime that cannot go unreported.