Yeah, that’s not an easy sentence. I had to look up a couple of words and double check that すら meant what I thought it did…
Here’s how I would break it down:
自分を殺すような相手、
“The sort of opponent who will kill you”. This is a kind of conversational tagged-on-the-front remark in a not strictly formally grammatical way, specifying the kind of opponent that そんな相手 is about to refer to.
そんな相手にすら傍観を決め込むほど日和見主義だった
“so much of a wait-and-see opportunist as to take an indifferent attitude to even that kind of opponent”. This whole clause up to ほど is specifying just how much of a 日和見主義 we’re talking about. 日和見主義 is one of those words I had to look up, but I think the meaning we want here is someone who sits on the fence and watches and waits to see how things develop, not committing until they think there’s an opportunity for them. Somebody who takes a bystander attitude to an opponent who’s out to kill them would certainly be an extreme proponent of a 日和見主義 philosophy.
~記憶はないのだが
…and in fact the narrator is saying that they don’t have any memory of being that much of a 日和見主義, but…
心はその相手の素姓など欠片も興味を払っていない
…emotionally they have absolutely no interest in that opponent’s identity.
So the narrator is basically saying that their lack of interest in this opponent isn’t an intellectual/logical decision (it’s not the product of a conscious choice to follow an -ism, a 主義, on their part), it’s just that as far as their 心 / feelings go, they don’t care at all.
By the way, if I ran into this sentence in an LN I was reading myself, there’s at least a 50% chance that I wouldn’t bother doing all the lookups and the careful attempt to break down the sentence structure, but instead would take 自分を殺すような相手 … 心はその相手の素姓など欠片も興味を払っていない and trust that that was enough meaning to be able to keep moving forward with the reading. (And as you can see from the above, that optimistic assumption would be correct.) This kind of sentence that trips you up by having multiple things you don’t understand all at once is usually not one you can usefully learn from by careful study – in my experience it’s better to pass over it, and wait until you find a sentence that only has one of the unknowns in it to learn about that unknown grammar/vocab/etc.