BH
I heard if a roidef is coming to try to kill you you should kill him first.
Where does it talk about this first in the Gemara and Halacha?
How does Halacha define a roidef?
And what if the Roidef is a Jew like if he went on the army of the opposing side?
It's found in many places, including Sanhedrin 72a.4, Yoma 85b.4, and Berakhoth 58a.15
As Curious Yid commented, Sanhedrin 73a.2 goes over the rationale in greater detail.
You can read the Mishneh Torah on the subject here: Mishneh Torah, Murderer and the Preservation of Life 1.6-1.16
