Meaning of 私がではない

Question

Spoken as part of a monologue by an anime character (cutting out parts irrelevant for context):

私にとっても、それは不快な事実だった。私がではない。　あれほど冷酷な魔術師だった男が、敵を助けたという事実こそが不快だった。

Is it a fixed phrase? What does it mean? What is が here? Just added modality (disappointment, surprise etc.)?

Answer 1

私がではない。あれほど冷酷な魔術師だった男が、敵を助けたという事実こそが不快だった。

The が in 私が is the case particle as a subject marker. 'I did not. / It was not I (who saved my enemy).' It was the 男(=切嗣) that saved his enemy (= me = 言峰), not the other way around, and 言峰 is saying that it was 不快 to him.

Source: http://www26.atwiki.jp/tmranking/pages/51.html

breakdown:
が= 格助詞/case particle
で= 断定の助動詞/assertive auxiliary, or copula
は= 係助詞/binding particle
ない= 形容詞/negative adjective

eg.
「XXがではない」 It's not XX (who does...)
「XXにではない」 It's not to/for XX
「XXのではない」 It's not XX's
「XXとではない」 It's not with XX

Answer 2

It means 'I am not', with emphasis on the I: watashi ga (emphasised I), dewa nai (am not). If you wanted to, you could put a na adjective or noun (no particles needed) between the watashi ga and the dewa nai to make

I am not (word)