According to the Torah, is it permissible to kill people in war? Or would that be violating the command given by G-d in Exodus 20:13 "You shall not murder"?

Thought this was inspiring and that I would share -

Isn't it unfair that G-d killed the slaves of the Egyptians because their owners forced them to work outside when G-d rained down hail and fire? (Exodus 9:21).

Why did G-d drown the Egyptians if they were fleeing and were no longer going to harm the Israelites? (Exodus 14: 25-27).

Shalom, my name is Claudia. I'm very excited to be here. Well, my introduction is not so easy because I represent everything that you probably don't like so much. I am german and I am a believer in Christ (since I was an alcoholic and faith brought me out). 

Why am I here? I read many biographies of Jews and I almost can't believe what my ancestors did to them. I am deeply ashamed of that. The more I read and the more I listen to Jewish music, the more I fell in love. I also saw that you have a thread where you learn Hebrew together, I already know the letters and 50 words.

Is the Jewish New Year (Rosh Hashanah) mentioned in the Tanakh?

Is the Lilith mentioned in Isaiah 34:14 the demon?

Can a Jew renounce being part of the chosen people and decide to continue worshiping G-d but as a noahide?

Why did G-d kill the firstborn of Egypt for the sins of Pharaoh?

Habakkuk 2: 4 says "The righteous shall live by his Emunah" What is a better translation for the Hebrew word "Emunah"? Faith or fidelity?