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Nihilist
Emotional charge
Using 'nihilist' can express strong disapproval if the listener values beliefs and morals. 
Avoid confusion
Don't confuse 'nihilist' with 'atheist'; atheists disbelieve in gods, nihilists disbelieve in all values. 
Not just negative
'Nihilist' sometimes refers to a radical questioning or skepticism, not only negativity. 