i want to start writing a song about a psychopath torn between his love for someone and his burning desire to hurt her.
im thinking about switching between a minor sound for the hurting parts and a major sound for the lovey parts given their respective sounds and emotions they evoke.
kind of like "low mans lyric" by metallica, verse is in A minor, and the chorus is A major

i have no prior experince with songwriting so give me all u got.
suggestions are more than appreciated
Well I've learned a few ways you can use keys to your advantage.

One trick is what you said with Metallica's song, the key changed from A minor to A Major. You can also use the relative major of your minor key, so if your verse was in A minor, you can use C Major.

Also using the dominant, or fifth, of the key you're in works for key changes as well.
So if you were in A minor, the fifth of A is E, so you can switch to E minor, or if you're in A Major, switch to E Major.

One last thing to remember= the 2, 3, and 6 chords of a Major key are always minor, and the 2,3, and 6 chords of a minor key will be Major, so you can use those chords in your minor/major key to give a different emotion without ultimately changing your key.

Hope that helps in some way.