Looks like your right drone nailed him as he turned through your stream and you hit his belly.
Happens all the time in FSO as bombers are in tight. I like the fact that Harpa was behind and high to you relatively. It was the turn action that did it.
I noticed that when you were firing above and behind, you paused your shots when your guns came across where he originally was located.
A good squadie ALWAYS feels guilt when they are responsible for their wingman's death. It is a sign of good discipline.
I'd chalk it up as a friendly fire ACCIDENT.
The Axis forces thanks you for the 20 points, however, I don't see either of you as being at fault for it. Just a learning experience for many many more FSOs to come!