In the real world WW2 planes smoked, exploded, burst on fire, spun horribly (so bad it would pin anybody inside) and pilots knew by looking it was going down, often breaking off to watch their prey impact into the ground.
It was quite clear when somebody was dead in the real war, most times.
This is a game. There are certain concessions for gameplay. Can't have complaints of "I shot this guy down and he killed me in a different plane before his first one hit the ground!!!" and so on, so forth. For gameplay's sake as soon as the person is back in the tower they are dead, and IMO that's when a kill message should be displayed. Kill messages are ALSO a gameplay consideration. You know they didn't have chat buffers in the real war, right?