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.

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.