there could be another possibility....heres my theory

that the only way to beat mangekyou sharigan, is someone who can use mangekyou sharigan, so what if itachi was in a stale mate with the other "user" and the battle was not going anywhere, so one tries to escape,(so if itachi was the escapee) and not wanting to admit a loss, itachi just said he killed every1 to give him a superior reputation, and no1 question about remaining survivors. thats just my input.

or itachi wasnt the escapee and wasnt worried about the other user and considered him dead as hes no threat