To turn against someone is to stop liking them and start disliking them and becoming unfriendly towards them.
Example of use:
1. The voters turned against their Member of Parliament when he lied to them.
2. The football coach denies reports he turned against his star player.
To turn someone against someone, or something, is to make them start disliking them.
Example of use:
Sasha’s parents turned her whole family against her boyfriend.
infinitive – turn against
present simple – turn against and turns against
-ing form – turning against
past simple – turned against
past participle – turned against