Screeeeeeeee Why did this take me so long- Anyway, Blood! Killer actually didn't randomly eat the black apple. After Nightmare gave it to Killer to protect while he was gone, the black apple grew interested in Killer's determination. After a little bit of (stab), the body was goop-ified. However, Killer still had control over his body and his tentacles...completely. This causes him to newly be the strongest bad sans. Even better than octopus. But, the apple stab body right? So all that red stuff- it isn't just a random color. From his name to what I just told you, you can tell what that red "stuff" is.
Thanks you to PJ_AU and Ask_Patchclaw for the originals! But hey, my hands hurt... So a bit of thanks to myself too!
