Nah, I didn't kill my old self, but since I detransitioned I've felt a lot like I had stuffed away my old self, like into a storage room somewhere far back in my mind, and I've dug around a lot in there to try to find her again now that I miss her. But I can't. The problem isn't that my old self is gone, it's that I'm still her but I've grown up and changed with the years and experiences I had while transitioning and whatnot for the past decade. 30 year old me simply cannot ever be the same as 20 year old me was, transition or not.
However, back when I thought I was a trans man, I saw everything through a lens of "myself as a guy," often unwittingly. Now I know I basically tried to "re-write" myself into what was a male persona, while I thought that persona was the real me that I was trying to liberate. Confusing times! Because I had just before my transition created a female persona that I was trying to force myself to become in order to escape being trans. I really wasted so many years on trying to force myself into either box of the gender binary, when both chafed against my soul.
Now I mostly just think of myself as a human being with an androgynous mind that just so happens to have been born female, and that's what I've always been regardless of how I've identified my gender or how I've changed my body. I see myself as a woman now only cause I made peace with my body being female, I don't really think I have a gender per se. I'm not a tiny teen with perky breasts and a high voice anymore, but that I'm now a hairy, flat-chested woman with a deep voice... is still my body and changing it never changed who I am inside.
Although of course it did change how I interact with society, and how society interacts with me, cause passing as male is obviously different from passing as female, and I never got the luxuary of being able to choose which of the sexes to pass as from day to day. And transitioning changed how I relate to my body. I went from very dysphoric to loving myself, but also from feeling whole to feeling amputated. I've grieved and felt liberated, comforted and torn apart. I've felt lots of confusion, frustration and many other emotions. So of course I've changed, going through all that.