Shinji might have wanted to kill them all but he didn't. In a sense that's very realistic: People want all kinds of things and may even speak how they plan to achieve them, or some of them, but often they end up changing nothing, only planning endlessly. So, in that sense the change in Shinji's attitude played only a theoretical role and little in practice, which is one reason why I said it was sidelines (I never said missing in my post if you check it again). In the end he never really hunted the parasites, only killed those that he happened to meet under unfotunate circumstances or that were hunting him (you pretty much knew that if Shinji met a parasite that was about or had just killed a human, that parasite wouldn't anymore see the next sunrise).