If I go up to my friend after we've been playing Nidhogg, grin and say, "Hey you son of a goblin, I'm going to slit your throat", he will probably laugh and carry on.
If I go up to that same friend upon meeting him for the first time in days, grin and say, "Hey you son of a goblin, I'm going to slit your throat", he would probably nervously chuckle and wave it off.
If I go up to a coworker who I do not have a close relationship with, grin and say, "Hey you son of a goblin, I'm going to slit your throat", he has every right to be afraid, even though my demeanor would indicate that I'm only joking.
If I go up to a random stranger on the street, grin and say, "Hey you son of a goblin, I'm going to slit your throat", he should probably be very afraid, no matter what I look like.
My point is: a threat on somebody's life, no matter the context and nonverbal cues, should be treated as credible unless both parties have a very clear understanding that it is not credible.