No, our society can consider it wrong as well. We don't have to give anybody a pass on abhorrent behaviour because 'they didn't know better'
People from hundreds of years ago may find many things we do abhorrent too. Does that mean we are wrong and they are right?
It will mean that. Morality is relative to time, culture and context. Ours will cease to matter to the future just as the morality of slaveowners has ceased to matter to us.
> the shock of the event they quickly labeled a “massacre.”
implies they did consider it morally wrong. Or do you mean only the society perpetrating the war crime should be the one to judge?
If the British actually considered it a war crime then why did they do the same thing to other people in many places around the world? They are only morally outraged because they are on the receiving end.