One wonders if the British were even half decent when you consider their long history of colonial and imperial oppression. One might conclude they were fully indecent, in fact.
The key is that they believed themselves to be decent, whether they actually were is hardly relevant to the effectiveness of the tactic. The genius of Indian resistance against British rule is that it forced the British to confront the gap between their stated values and what it took to maintain imperial control over India.
Amritsar caused outrage in Britain and changed the whole narrative. It took time, but within a decade British government had lost all moral ground and British attitudes had shifted.
I don't think the point was about half decent vs. a quarter decent.
As evil/bad as the situation was; non-violence only helps if someone loses more than they gain by just killing you. In this case it was by losing the moral high-ground that was valued by some British people at least. Sure it probably wasn't decent people among the perpetrators, but the pearl clutchers back home.
Mainly just clarifying to the parent what Santosh83 meant. I think it's not about x% decent but rather x% decent able to sway ruling class decisions based on the optics of attacking non-violent dissidents.