I think that yes, it is true that nihilism as a way of looking at things is very immoral. It does not do what the majority wants, and in many ways goes against the moral beliefs of the majority. I am not one to support it, and think it is dangerous.
Nihilism, by definition, has nothing to say about morality. Many nihilists use their nihilism to defend the position that there are no morals, or something against the mainstream view, but this is a contradiction because it's defending the morality of no morality. According to true nihilism, hardcore Christians who believe in absolute truth are no more immoral or unjustified than anyone else. Nihilism is a useless belief, that does not have anything to say about how one should live one's life. It doesn't even say you shouldn't care, or do nothing, or live with no morals. It says nothing. Because of this, nihilism has no morals, and is immoral insofar as you believe morality exists at all, because nihilism isn't in opposition to anything, including immoral action.
Not people's beliefs. Only when people act on those beliefs is it a good thing or a bad thing. Of course, that is left up for each individual interpretation Nihilism is the belief that people made up morals. That there really is no such thing as morals. Its not immoral to think that way. It would be immoral for people to violently attack someone, and think it is OK, because they do not believe morals exist. That is determined by society as a whole. There are laws that everyone disagree with. We still follow them because we don't want to face the consequences and many of us understand that the majority of the population agrees with these laws. There is other ways to fight them than breaking the laws. Plus it's common decency to respect other peoples different viewpoints.
Nihilism is the belief that life has no ultimate purpose or meaning. People can take that thought in many directions, come to many conclusions, but the idea of nihilism itself is not immoral.
Are some nihilists immoral? Sure. So are some christians, mulsims, hindus, buddhists, etc. Every ideology has some pretty bad conclusions associated with it, and some bad actors in their groups. Now, if you were to say that a certain group of nihilists have formed an immoral ideology with nihilism at its core, I would be with you.
It is also possible to use nihilism to get to deeper moral reasoning, asking questions about what we really value, and what is in our best interest, without the interference of ideas like a higher power or higher purpose. If your purpose is what you make it, then doesn't that actually give us our biggest starting point for reaching common ground and agreeing, objectively, on what is best for the individual and society as a whole?
Basically, religious arguments fall flat because you can't prove a higher power, leaving everyone to fill in the gaps with whatever was written down based on questionable divine revelation. Nihilism does away with this to ask the question of what we truly do value, rather than what religious texts say we should value.
Blank slate morality, starting from what people want as a social group, vice the voice of an unconfirmable, all powerful deity that seems to speak in the voice of corrupt people?
Nihilism cannot be immoral as a nihilist does not believe in morals. This is like saying "Are you against unicorns?" No I'm bloody not because unicorns aren't real in the first place (although I do still hope that one day we'll find one). And even if let's say my argument doesn't hold. Nihilism suggests everyone and everything are a 0. This means everyone and everything is equal. This results in 0 discrimination. Of course people have preferences, but those preferences will no longer be biased as we view everything as nothing. Some say because you are a nihilist you want to kill yourself or you are more likely to kill others. I don't understand why. I don't understand why people, knowing they are 0, would not want to be a 0x10000. Why can't you have an awesome life, just because your life is meaningless? All humans want to be happy and if you're life is meaningless why not try to live for others. It's not nihilism that makes people "immoral". It is simply people's lack of strength and will to live for others and live an awesome life filled with great sensory experiences. This would however mean that if you like killing people then do it ... And yes I am saying that. But in a world where everyone understands they are nothing, yet still want to have a joyful and amazing life there would be no murderers due to the lack of environmental factors which usually lead people to do very degrading things to other people. It's not nihilism that is the problem. The problem is the nihilist's lack of imagination of how to use nihilism in a way to help humanity become one of the greatest species in the Universe. Nevertheless, if you believe that humanity will not change then do whatever you want because there are no morals, no right or wrong, no good or evil and the Universe does not punish the wicked and it does not reward the excellent. See, even this makes people feel free and alive? This makes people feel good, and when people feel happy they tend to, I don't know, ALWAYS MAKE OTHERS HAPPY (I speak from personal experience. I've become a more loving and more pleasant person to be around lately). In the end what we always forget to mention is : even if there are morals what is their backbone? Because what is chaos for the fly is normal for the spider. Morals themselves are immoral relative to each other and relative to the circumstances they are projected into. There is no black or white, only grey.
Amorality is different from Immorality, immorality to go against morality, amorality is to have no morality. The two words might sound similar or to have the same meaning, but amorality doesn't imply someone has the intention of going against a defined morality in any way. I am a nihilist personally, and this is a common mistake people have made in thought, usually saying 'you're a nice guy, how can you be a nihilist?' like nihilistic beliefs automatically make a person a jerk, this isn't any more true than the statement of having a faith makes a person automatically saintly.
Nihilism is the belief in the absence of being, in the lack of choice, the lack of will. If there is no choice, no will, there is consequently no right or wrong decisions to be made. There is no path to choose, ergo no wrong way home, no direction astray.
While nihilists hold the belief that there is no inherent morality, they are not inherently immoral. Nihilists realize that nothing in the world has any value or purpose, but at the same time a lot of them understand that in order to survive one must act with common decency to other people, because just like the rest of the public, they rely on other people to live. One cannot say that all nihilists are evil just as one cannot say all Christians are Christlike or all Buddhists are vegetarian. I myself am a nihilist/absurdist and still hold all basic human rights to be fully valid (freedom of speech, religion, right to life, etc.).
It is not helpful to focus on and overanalyze the reasons people have for moral behavior. It's reminiscent of the middle ages where people assumed atheists had to be immoral because they didn't have God to tell them to behave. Nihilism just says that there is no ultimate, solid, objective value. That doesn't mean you can't have subjective values. Nor does it mean we can't based on mutual considerations and mutual interests come to some logical conclusion about the sort of morality to live by and the sort of morality to push in society (which depending on circumstances may even be two different answers). People will come to different conclusions here and sometimes there will be social conflicts but the only way we grow and change and evolve is with these conflicts.
I don't really think there is any credible argument to be made that nihilism as a general concept or what not can be considered immoral. It is a way of thinking about things and does not really seem to extend much further beyond that. It isn't fair to call the entire concept immoral.