Ok, here are a few lame attempts of mine to figure this out.

The gunman had intended to kill Henry and only realised he got the wrong man by reading the death notice in the paper. He rectified this by killing Henry soon after.

Or the gunman had intended to kill both and realised that one had survived, so killed the remaining witness later.....

Or perhaps Henry had donated an organ to help save John's life and it had failed and proven fatal for both for different reasons....

lol, I'm sorry I am no good with these riddles!