Supergirl, Season 5

Supergirl:
Lena, you didn't kill Lex. I... that was me. I... I watched him... I saw him fall.

Lena:
You saw him fall, but did you see him land? Did you see him die? I did, and let me tell you, it wasn't pretty.

Supergirl:
That's impossible.

Lena:
Anything's possible when you're a Luthor. Lex used his transmatter portal watch. I knew exactly where he'd go. The cabin we loved when we were children, so I was there waiting for him. I had the gun ready, loaded. I could feel the weight of it in my hands. Every fiber in my being rebelled, but I... I didn't want to do it, but I knew I had to because if Lex lived, the world wouldn't be safe, my friends wouldn't be safe. So I forced myself to pull the trigger. I shot my own brother in the chest. His final words to me were that I was a fool, that my best friend, that every friend I had was lying to me. With his dying breath, he told me that you were Supergirl.