As a long-term mod who has put a lot of effort into improving "my" communities*, comments on your final paragraph, especially as I see from your profile that despite having a seven-year account, you don't seem to be a current mod anywhere, so some of this might be news :

Turn off the downvote button. - this sort of works for desktop old-reddit users who accept CSS, who are now a minority of users. There is no way a moderator can turn off the downvote button for users redditing via an app, or via the new desktop design, who (both groups together) are now easily the majority.

Try to only ban people and delete posts for a lack of civility/respect or outright bullying/abuse, rather than unpopular views - false dichotomy, most of my bans are to people who refuse to read sidebar rules, and spammers. Most bans are temporary, though, and we've had really good ongoing contributors who have been through temp-bans. Making most first bans just a week long, if there is any possibility that it was just someone having a bad day, rather being a dedicated asshole, would be a good piece of advice.

Perhaps use AutoModerator tools to encourage open and civil discussion - I really have no idea what specifically you mean by this. Can you elaborate here?

The second half of that paragraph, I haven't quoted because it's excellent and true.

*(except this one. I'm still a mod here, but it's the only sub I mod that I don't enjoy any more, because it's become awfully political and attracts a ton of hyper-partisan arguments ever since Trump became the nominee, and we could no longer reasonably exclude the-donald from our topics. I'm not even American. It shouldn't be my job to clean up vomit from your broken country. But I still have just enough love for the subreddit from the old days left to stay and occasionally clean up vomit, but that's all it feels like most days, except the rare treat of when we get a good well-written post like this, so thank you.)

Try to keep in mind that there are downvote bots operating in nearly every single sub and that vote counts are not accurate and are fuzzed to deter spammers. So it's important not to get caught up agonizing over a downvote that may or may not exist AND may or may not have been given to you by any human being.

I have been active on different SO/SE forums for quite a while.

The sites here aren't forums, they're tightly curated Q/A resources, more akin to Wikipedia than a social network.

However, I have barely seen the same behavior from other SO users. Consider this post for example. There I have asked a question about compiling software on FreeDOS and I didn't ask it on SuperUser because I speculated (wrongly) it will include some programing.

The question was closed, and the close message gives a very useful pointer for you, namely:

"Questions about general computing hardware and software are off-topic for Stack Overflow unless they directly involve tools used primarily for programming. You may be able to get help on Super User."

Which tells you quite a bit about what went wrong with your post. Downvotes don't come with explicit messages, and this has been brought up and declined more then a hundred times already (no, really), but they do have a description on the privilege page and the arrow:

This question does not show any research effort; it is unclear or not useful

From which you can extrapolate quite a bit.

As soon as I wrote this I experienced a tsunami of downvotes and flags on my posts and former posts! someone even took the liberty to edit my post and remove this criticize. So here are the points I want to discuss:

That's regrettable but also understandable. In your message you basically accuse people who donate their free time and expertise to keep stack overflow clean of refuse and off topic questions of being toxic. Being that this is tagged with psychology, one of the universals of human psychology is that people dislike being accused of things they find reprehensible but haven't done. It''s therefore unsurprising that editing such a paragraph into the question would draw negative attention.

what exactly happens in the mind of a person causing him/her to downvote others without giving them the chance to learn why? and why when this behavior is criticized they tend to do it even worse? Has there ever been a psychological study on people's behavior on social media and how they try to punish/silence others?

When you curate posts on Stack Overflow, you see a lot of bad posts. Like, 90% of what you see in the close votes or new posts queue is bad or off topic. The strain of explaining the same thing to 1.000 people making the same mistake day after day makes people curt in their responses. The SE system has mitigated this by reducing human interaction: close reasons are largely canned, and commenting about your downvotes is discouraged.

I disagree that this has anything to do with silencing or punishing others, that is your interpretation into it from not knowing exactly how it works. This here site is not a social media platform or forum, it is a (supposed to be) tightly curated Q/A resource. You wouldn't accuse people of silencing others on wikipedia because your contribution to an article got rejected, for instance. It's in the same vein here.

Regarding for why making wild accusations draws additional negative attention, see above.

Why SO/SE still has the feature of anonymous downvote without explanation. When there is a flag option what is the benefit of having downvote at all?

This topic has been discussed, the idea rejected, and the topic discussed hundreds of times on Stack Overflow and on here. So much so, there's a FAQ dedicated to listing the arguments. You'll note that FAQ has 60 undeleted linked questions alone.

why there is no option to move questions across these SO/SE forums if being "on-topic" is so important?

There is, in limited circumstances, it's called migration. It's not often used, because usually questions have to be reworded / reworked significantly anyways to fit on the scope of another SE site, and usually these questions are poor to start.

There are some great people on reddit. There are also some colossal douchebags. I think depressed people (or perhaps all people) tend to view things like reddit and facebook as a means to gauge social acceptance/rejection. I think the anonymity gives people a sense of security, so sometimes they conduct themselves in an unsavory way without the fear of facing social consequences. Don't measure your self worth based on what some semi-sociopath neckbeard thinks. Nobody can hold opinions that everyone 100% agrees with 100% of the time. Just stay true to yourself and your beliefs, whether someone else agrees with your beliefs is their problem, not yours. Also, I've seen reddit downvote some truly great content, while at the same time upvoting complete and total crap. I'm not sure I would want reddit to agree with all of my opinions.