Coke Zero is bad for you just like other sodas when consumed in excess.
Even though Coke Zero is sugar free it also contains sugar substitutes which are not really healthy or healthier than regular sugar.
Coke Zero Sugar contains artificial sweeteners, aspartame, and acesulfame potassium. The U.S.
Food and Drug Administration has approved these and considers them to be generally regarded as safe (GRAS) although they can still be unhealthy when consumed in excess.
If you want to drink some soda and still lose weight then Coke Zero is a better option than regular Coke or other sodas.
Or if you're diabetic than drinking Coke Zero is better.
You should still drink other healthy drinks such as water, tea, juice etc to keep your kidneys and body healthy.