Perhaps I missed it, but when did we learn who killed the Ajira passengers?
Well, you're right, I guess it could have been some unknown third party who killed them, but I don't think that's very likely.
Regardless of what Flocke has said, I think it's entirely possible FLocke is somehow responsible. Remember at the end of last Season that Flocke told Richard that after he saw Jacob that they would need to "take care of" the ajira passengers and when Richard asked what he meant by that FLocke said "You know what I mean".
I could certainly be wrong, but I think Flocke is as likely a candidate as Widmore.