In the dictionary:
Dead: no longer alive
Her mother had been dead for ten years.
Her parents were long dead.
The man was already dead
So, what is the difference between "He is dead" & "He was dead"?
If we choose other adjective "nice" for example, then it is so easy
He is nice: At this moment he is nice
He was nice: He was nice in the past but we don't know if he is nice now
But: He is dead: At this moment he is no longer alive, but when he is dead, that event was already in the past even if it only happens a few seconds ago
Or we can say "He has just been dead" seems accurate
He was dead: He is no longer alive in the past.
When to use "He is dead" and when to use "He was dead"?