She was always nuts. She only got into the idea of being nice to people and protecting them when they started praising her and parading her around on high.
She freed slaves so they would worship her.
Exactly. From season one she is shown as manipulative. Back then it was a survival mechanism but she also used it to her advantage to further long term goals when she made Drogo fall in love with her. She loved him too but make no mistake - it started with her wanting control over his Dothraki.
Everything she did was not born out of altruism. Free the slaves? Sure, noble thing but you also gain a workforce and powerbase that is beyond loyal to you. Gain control of slave eunuch soldiers? Sure, noble thing to free them from their slavers but you also gain an elite fighting force that is also utterly loyal to you.
She was always a calculating person and the show never hid that, most people just stopped at the surface and only saw the positive sides but once she arrived in Westeros the veil started slipping and finally came off when she burned the Tarly's needlessly and allies started to abandon her. So her final turn during the assault on King's Landing was just the final step in a long journey.