Zayn Malik vs. 1D's Louis Tomlinson -- Bad Blood

Zayn Malik's exit from One Direction apparently wasn't as friendly as they wanted us to believe ... because Zayn and Louis Tomlinson are publicly trash talking each other.

Zayn just posted this heavily-filtered photo of himself with record producer Naughty Boy. Louis jumped on it, tweeting, "Remember when you were 12 and you used to think those Mac filters for your picture were cool haha! Some people still do HA!"

Zayn was locked and loaded, firing back, "Remember when you had a life and stopped making bitchy comments about mine?"

People connected with 1D were insisting to us when Zayn left ... everyone was cool about his departure ... which made no sense.

Harry ... your move.