Totalitarianism is the attempt to control thinking.
The usual excuse is that this is the best or most effective way of controlling behaviour; but in the end that is just an excuse - and the real and strategic motivation is to control thinking because that is what is wanted.
Totalitarianism has nothing necessarily to do with violence (as Aldous Huxley perceived in his Brave New World of 1932 - and to equate totalitarianism with violence was an error by Orwell). For totalitarianism 'whatever works' is the guide.
Thus we now, in the West, live in a highly totalitarian society, in which most people's thoughts are controlled most of the time - by a combination of indoctrination during childhood and youth, the unified-linked bureaucracy of the government and the workplace, the mass media and its addictiveness, and a legal system which explicitly includes thought crimes (what else are 'hate crimes'?).
There is a combination of filling people's minds with approved thoughts, and excluding disapproved thoughts (by direct exclusion, and by simply driving them out with stimuli, noise, and the prevention of detached solitude).
There is very little concern about this state of affairs - and indeed it seems that many or most people positively approve of totalitarianism so long as it is 'in a good cause' - thus our current secular Left totalitarianism is okay because people agree with its goals; and indeed ost people apparently want ever-more totalitarianism so that their goals may become realised.
So why is it bad to control peoples thoughts, if the cause is good behaviour?
Well, totalitarianism isn't bad for atheists and secular people generally, which is why our society is as it is; since the aim is merely 'good behaviour' in pursuit of goals like comfort, peace, prosperity and amusement.
In practice (and ignoring empty words), all non-religious people approve of totalitarianism.
Also totalitarianism isn't bad for most religious people in most religions - which is why there have been many religious totalitarian societies that regard thought-control as a legitimate means to religious ends. (They were usually limited by their technology, and the degree of support; but the totalitarian aim of thought-control was explicit.)
Totalitarianism is only really 'a bad thing', an evil, for those Christians who view motivatin as primary; who regard freedom or agency as central to salvation and theosis - such that only a truly free, creative, and divine decision of our truest and deepest selves is valued by God; and our actual behaviour is secondary, and indeed (since behaviour may be coerced) almost orthogonal to the divine purpose.
Any system of any kind (whether political, in the workplace, the church or the family), of any aim, that tries to control thought, or to prevent agency - any system which tries wholly to occupy the mind - is therefore evil to this kind of Christian.
This kind of Christian - the kind that regards agency as non-negotiable and central to the faith - would regard any and all types of totalitarianism as evil; no matter how kind, how non-violent, how materially successful; and no matter how happy was the resulting society. After all; Huxley's Brave New World was almost certainly happier than any society of recent centuries.
If happiness is the number one priority; then totalitarianism is the likely means to that end and will sooner-or-later be embraced - and then (as now) the only genuine dispute relates to who is in charge of the totalitarian system.
And that question, in a nutshell, constitutes the entirety of modern mainstream politics.