Summary: Clara only has so much control over her stolen Dalek shell.
It’s the goal of every Dalek to exterminate anything that isn’t a Dalek. It makes for a rather monotonous existence but Daleks don’t care because they can’t get bored or dissatisfied with their lot in life; the only emotion they’re capable of feeling is hate, it’s hardwired into their bodies and their shells so completely that it defines them.
Inside the Dalek’s casing, hooked up to it via electrodes attached to her head, Clara has control over its movements, but she can’t change its basic nature. When she says the words ‘I love you’, all that comes out is “Exterminate!”