True, the DoI did not establish a government.
And if a human has the moral right to fight for freedom, don't other humans have the moral right to help them do so?
According to the Bible, those in slavery are to treat their masters with respect - not murder. But just how many murders are justified?
Also according to the Bible, and to Christ specifically, Christians are to respect the government and not rebel against it. Yet Americans chose to rebel against the British King, their legitimate sovereign, on the grounds that he was infringing on their liberties. Presumably you believe they were right to do so.
Yet you think black men and women, sufferers from infinitely greater deprivation of rights than King George ever considered, had no moral right to rebel?
But just how many murders are justified?
This is a question of how the rebellion should be fought, or indeed of whether the cost of rebellion is more than it is worth, not of whether rebellion is morally justified. OTOH, if the masters do not resist, which they not have a moral right to do, presumably there would be few deaths.
It is interesting how attitudes have changed over the centuries. To a modern person, Spartacus is a hero, as the movies made about him have shown. To people of antiquity, he was a great villain and monster.