If you turn against someone or something, or if you are turned against them, you stop supporting them, trusting them, or liking them.
A kid I used to be friends with turned against me after being told that I'd been insulting him. [VERB PARTICLE noun]
Workers may turn against reform. [VERB PARTICLE noun]
Working with the police has turned me against the use of violent scenes as entertainment. [VERB noun PARTICLE noun]