War Machine -- I Could've Shot Christy Mack ... If We Were in Texas

War Machine thinks his ex-gf Christy Mack got off easy when he beat the hell out of her -- because he could have flat out killed her and gotten away with it if the attack went down in Texas.

War Machine actually believes he shouldn't have been charged at all ... according to a jailhouse letter he recently wrote to a friend. TMZ obtained a copy, and in it he claims Christy is throwing him under the bus to protect her "image."

The ex-MMA fighter says Christy's friend Corey Thomas ambushed him when he entered her home back in August -- and adds ... "Even if I wasn't ambushed they still would have got what they deserved! In Texas you could have shot them both and got off with that 'passion crime' s**t."

He's no lawyer -- fact is you can commit a crime of passion in any state, and it's not a free-pass to go killing people.

War Machine's facing 32 counts ... including attempted murder, battery, kidnapping, and sexual assault.

In the letter, he says he plans to sell "#FREEWAR" gear to pay his legal bills.