Though I suppose that wasn't the answer to your question, but rather some background information.
You asked what Fi "feels" towards injustice, and I must tell you, that Fi is simply NOT externally defined in that way.
If it is injustice perpetrated by the Fi - then it is quite possible that no guilt or remorse is felt at all.
Generally though, when I perceive events to be unfair I find I "feel" quite badly about it, occasionally wanting to sympathize with the oppressed party.
Alternatively, one might exhibit schadenfreude in very inappropriate contexts.