Using 'nihilist' can express strong disapproval if the listener values beliefs and morals. Calling someone a nihilist can be offensive; use it with caution.
Pro Tip 2/3
Avoid confusion
Don't confuse 'nihilist' with 'atheist'; atheists disbelieve in gods, nihilists disbelieve in all values. Unlike an atheist, a nihilist may reject even secular systems of meaning.
Pro Tip 3/3
Not just negative
'Nihilist' sometimes refers to a radical questioning or skepticism, not only negativity. As a nihilist, he questions all societal norms but doesn't necessarily dismiss them.