Those are actually a whole lot of questions and the answers heavily depends on unstated parameters. Like morality is just about "doing the right thing". But what is that, who defines that and who judges whether an action fits the definition. And in that judgement you could again take either the perspective of the person acting or the perspective of a bystander looking at the result of the action.
So if they actually believe that nonsense, craft a morality based on it, which then prescribes things like "assaulting other people is good" and then act upon that prescription, then they would be technically acting "moral"... that is towards their own (and hopefully exclusively their own) morality.
Seriously if you believe X is good and then do X, because you believe X is good, then that's pretty much the definition of acting moral. It's a case of logically valid reasoning based on bullshit premises that are treated as axiomatic. Like that's a trivial and not terribly interesting case.
And from the point of society the judgement of their action should also be pretty trivial. It's a bad action with regards to the result, it's widely obviously communicated that it's a bad action, it's done deliberately and not by accident ... so from the point of society the action is obviously immoral. And as that believe prescribed and thus "necessitates" the immoral action, it's not much of a stretch to argue it's immoral as well, at the very least by it's result being bad. You can further differentiate between the logical reasoning and the flawed premise, but either way for any moral framework that not subscribes to that flawed premise, it's pretty bad.
Now your actual question seem to be whether the person is culpably guilty. In the sense of having an awareness of the immorality of the deed and of the belief and thus as to whether they themselves are responsible for the evil being done. And again yeah pretty much, they likely were aware of the existing laws and regulations and made the deliberate choice to put their own moral compass above that, which is a violation of the monopoly of morality that the monopoly of violence seems to claim and thus he is fully culpably guilty of the crime.
So if the question is really just about whether it's immoral (from the standpoint of society), yes it is.
The crux is not to label the action or the believe the crux is what to do with the culprit. Like the goal of the institutions concerned with creating and upholding morality, is to encourage "good" actions and prevent "bad" actions. And for example punishing people for actions that were outside of their control, like accidents, is something that is possible, but pointless. Because the individual involved in the accident is not the cause of the accident they are so to say just the last domino piece.
So even if we assume the individual to be without free will and only look at these interactions as chains of events, as domino pieces pushing the next in line, we could nevertheless distinguish between whether a piece is at the beginning or at the end of the chain and we can look at different chain of events and determine the different options and the likelihood of them occurring and thus make predictions about cause and effect and thus see which effects are unavoidable and where we've got a handle on things if we changed the trajectory of the domino piece or our trajectory to not be made one.
Though regardless of whether human beings are deterministic or possess free will, it's still probably better to model them as possessing free will as, apart from a few edge cases the range of human options is so vast and unpredictable that it makes sense to thing of them as subjects rather than objects.
And of course thought, feelings, emotions, believes, anxieties, hopes, dreams, desires and whatnot are domino pieces in the chain of cause and effect and can be labeled as good or bad. It's asinine to think they aren't, the crux more that they are difficult to work with.
They are hard to detect and quantify, up to the point of "irrationality" (not really, but an observer might have a hard time figuring out the rationality in it) and they can be fleeting and changing on a whim all while possessing the strongest possible justifying powers that a human being could subjectively experience.
So if your society aims at equality and a mutual coexistence of equals then it absolutely could and should see believes to the contrary as problematic regardless of whether they are "free willed deliberate individual malice" or "a force of nature". Though what constitutes an appropriate reaction to it would probably be difficult.
It's not "just an opinion" but a clear threat to the fundamental values of society and beyond a certain point it can become a self-fulfilling prophecy especially if the racists have the power to frame the debate. Like if you harass people 99 times and they fight back and you only show that with no context you can retroactively justify the harassment and stuff like that.
So no that is serious and can and does do a lot of harm. The problem is rather where is it coming from what attracts people to it, what's the grain of truth used to sell the lies and so on. So do people extrapolate individual experience to a whole group of people, is there a general atmosphere of fear that is bred and nurtured, is there misinformation being disseminated. Is there an identifiable source, is there a group that is recruiting or is it more of a general vibe in the population? Are people actually motivated by the bullshit or just follow along because you have lots of outcasts who want to be part of something and are willing to believe negative framing about people they that they don't like or don't care about?
The problem is usually that the reasons for becoming a racist are multifaceted and personal, while the effect of it is monolithicly bad. And if the demand of your group is something unacceptable like the discrimination of another group then accepting it is impossible, even considering it rationally is problematic but ignoring it also frustrates those sporting that message, so it's lose lose and at that point you've probably have to consider them an enemy and go for containment. Which is not great at all and not a long term strategy.
So if it were to be that simple like in your example it wouldn't actually be a problem, yes treat that believe as immoral and that's it. But usually live is more complicated.