An inspiring collection of photos, but I'd bet it attracts little interest outside a few select, irrelevant circles. One of the most heartbreaking thing about the massacre is not just that the mass murderers who ordered it were never held responsible but that today it barely even tarnishes their reputations, while the memory of their victims has been thoroughly suppressed.
I'm going to take a kind of despicable position. We don't know what would have happened had the protests been allowed to continue. China is a vast country with mostly poor, uneducated people -- especially so in 1989. Look at what happened in Syria. Look at the Arab spring. Is it just to kill hundreds, to prevent the slaughter of hundreds of thousands? If the answer is no, never, I don't see how we can have states at…
A question like that can often be used to justify extreme exercise of state power -- witness raids on armed vigilante groups in the US -- but that's not quite the right question in this instance, which was a peaceful protest for democratic reform. As you say "we don't know what would have happened" here (and of course the Arab Spring couldn't have been a cautionary tale as it was decades in the future).
So the question in this situation is really "Is it just to kill hundreds, to prevent a hypothetical bad outcome you can't possibly predict, can't prove and which even if it happened you could mitigate in other ways at that time?". To which the answer is palpably "no".
In fact, it's a hell of a stretch to say oppressive dictatorships ever carry out actions like this "for the good of the many". They generally just carry them out to maintain their grip on power, which is an end in itself for them. But not one that justifies massacre, in the main.