well, my DVR didn't catch Hawaii Five-0 tonight (episode Lapa'au), but I still have one thing to say:
I think I know who did it. My three canidates are:
(first a side-note disclaimer: I can't find how to spell the man's name. Makato?)
Canidate One: Wo Fat. This is probably what the writers will end up confirming. If it was him who killed the Imperial veteran, then Joe and McGarrett likely said the two possible reasons why Wo Fat killed him: either Wo Fat found out Steve had met the man, or the veteran was already on the to-get-rid-of list.
Canidate Two: Joe White. Thanks to Wo Fat attacking him, Joe now has the perfect way to deflect suspicion: say "Wo Fat came to kill me, must've gone here too"...which he did. Why would Joe want to kill the man? I can only guess...took the opportunity to get rid of someone who had been a thorn in his side for however long (honor and-or revenge might also have entered into it)....in which case, why wait until now? Well, Joe might not have had the opportunity before - most of the time, he hadn't been in Hawaii, and when he was, the perfect scapegoat didn't arrive until Wo Fat redecorated Joe's kitchen.
Canidate Three: Someone else entirely. Hell, it might even have been a robbery gone wrong. The point of this option is that his death might not be connected at all to Steve McGarrett, Joe White, or Wo Fat.