Your second point is, of course, true, that's why you establish a system with enough checks and balances and a horizontal bottom-up democracy so that those that do wish to subvert society have as little impact as possible. Here's a reddit post that kind of describes this idea and how it works against people that want to kill the revolution:
What prevents military junta to take power?
Well a key idea in my mind to preventing the military from turning against the people is to change where its power derives from. In the modern day idea of nation-states militaries derive their power from the government, that is the army is an apparatus of the state and the "chain of command" ultimately answers to the political interests of those in power. In this model, a military will not act in the interests of the people because it, competing against the other "arms" of the state, will seek to gain power over the other institutions to get more funding and more influence within the system.
On the contrary, in a democratic socialist society, the key to keeping an "army" (which would take the form of self-defence forces more than a traditional military) will be responsible to the community which it serves? Through which mechanism, you ask? Well, let's take Rojava, for example. In Rojava, the self-defense forces are organised into "military councils", with each region/city having its own military council. The "political" bodies here are elected by the local community, and hence things such as the resources and usage of the "army" are ultimately dictated by the people of the area which it operates in.
What stops whoever gets elected here from seizing power? Well, the soldiers within these military councils are generally members of the local community. They know the city, they know the people they are not going to shoot their own families for someone they barely know. The officers/generals are elected by the soldiers directly and can be recalled at any time, so if a general (who is also part of the local community) does for whatever reason try to follow this command to shoot up the local population and take control then he or she loses their power base, that is the soldiers that come from the local community, and he loses his position. This creates a clear mechanism through which the power of the armed forces is curtailed by, firstly, the democratisation of the armed forces, and also the decentralisation of the army, that is it is not an apparatus of the state but it is more directly a tool of the population to protect it from external or internal threats.
So what stops the different military councils from infighting? The answer to that is that the different military councils are not completely independent of each other. While they do act with autonomy and do have separate command structures, when needed (for example fighting ISIS) there is a superceding organisation, in our case study this is the SDF. All the military councils are part of this larger organisation which allows for local commanders to come together and organise larger scale organisation. Again it is entirely democratic and all power comes from the bottom-up, so if people are ordered to act against their community then there is nothing forcing them to do so.
what prevents totalitarian religious sect to take power (see what scientologists did in the USA)?
There are a large range of mechanisms which can keep extra-political forces like religion/ethnicity from asserting control over the system. Let's start with the first one. Whilst local policy decisions are made are a communal level, changes in law require the approval of an executive council. This is a representative system in which members are elected by the various areas of the society. This can act as a "check" against religious forces because even if one area is really religious they cannot implement religious extremist policies because they will get voted down by this executive council. Let's say, for example, one bit of the "area" is much more conservative and much more religious than the rest and so tries to ban women from being able to drive. This is just an example. Let's say that the communes of this local religious region all agree on this so the law is put up to the executive council to be voted on. Because the other areas, and their populations as a whole, will also have a say in this decision they do have the ability to stop it wherein it restricts others right to self-determination. So how can this be guarenteed to happen? Well one way that it can be ensured to be voted down is through a constitution. This constitution can gaurentee rights such as religious freedom, equal rights for all genders/ethnicities, right to self-determination and so on. If no one body has the authority to act against this constitution then even if an extremist view does gain legitimate popularity there is no legal basis through which this "tyranny of the majority" can occur and so minority rights are protected.
So how is this constitution enforced? Well ultimately any society does in truth rely on the will of its population to enforce the "social contract" of the society, and if no-one believes in socialism then ultimately socialism can't occur. This is a truism for any society so goes without saying, and I am going on the assumption that there is a solid degree of support for the democratic socialist society which I am envisioning since socialism relies on self-determination and democracy to operate.
Still though, even with organised opposition to constitutional freedoms, there are ways by which the freedoms can be protected. One of these are indeed the self-protection forces. Since the self-defense forces are independent from any political body such as the executive council, the political choices made by the council are independent of how this self-defense force operates. Even if somehow a religious extremeist force takes control and passes a law that restricts the freedoms of women or whatever, that political law will not be enforced if the population perceives it to be unjust, as the self-protection forces, since their power derives from the local population, will as such act in a way that supports said local population, even if that means disobeying the laws passed in the executive council.
These different branches of power can counterbalance each other even if one gets corrupted; keep in mind that these three-the communes, executive body and self-defence forces are not the only avenues of power, there are also trade unions, women's councils, farmer's councils, co-ops and so on. All these different bodies do not act towards the same centralised body but are autonomous groups of democratic bodies whose power derives from different areas, but also do have means by which they can co-operate, meaning that conflicting interests will not mean they end up competing and conflicting against each other. If one branch becomes corrupted then the others can group up and supercede its authority before its influence can spread to others, I guess. The rights of minorities are thus protected from extremist groups. I guess I haven't written this perfectly so if you need clarification I am happy to provide it.
what prevents one charismatic leader with his party to take control over everything?
The answer to this is pretty much the same-even if a group led by a charismatic guy gains control over some of the political apparatus then there are alternative ones which can act as a check against the influence of the institutions which are under the control of this charismatic leader.