Doesn't even need that. If Boris is in the foyer, I'm answering the front door. If push comes to shove and I'm killed at the front door, they already know a threat exists. Then they enter the house and attempt to detain Boris.

I just wonder where the line is. Is it intent to harm. So if say the thing was a gift, and they didn't know it was explosive. Would that absolve the person of guilt. Maybe it goes to manslaughter in that case?

There's a lot of reasonableness and average person would expect in law, but needing a supersonic bullet to hit it is well out of reasonable territory.