We have decided some brain quirks are disorders (and get accommodations, as is compassionate), whilst others are flaws (and get slurs). But no one picks their hardware. You cannot earn a better prefrontal cortex or deserve a calmer amygdala. Nor does one get to pick the environment they are born in, which will inform their choices later in life. Even the capacity to “learn better” is a roll of the dice, some brains start the race with sprinting shoes, others with lead weights.
So when we call someone stupid, lazy or insane we are not describing a choice, but simply announcing which kinds of unlucky we’ve decided are worthy of scorn.
I admit that website is annoying to navigate, and yes a lot of the content is as videos.
https://www.cnvc.org/learn/nvc-101 Perhaps this.
If you want to read, this covers roughly the same principles: https://pastebin.com/ZHhS044M
Banning words is not the point. As you said, a word can have many meanings. I am calling for understanding what you yourself want to communicate, and what you want to achieve with that communication. If you truly just want to insult people and that is the goal, then yes. It is indeed most effective to just throw a slur at them.
But if you wish to bring about some good (human well-being) in the world, perhaps learning how you can communicate to someone that their actions are harmful, without putting them on the defensive.
Also, if you wish to call someone stupid because they are behaving like a fool, you need to first understand what their goal was. Perhaps their goal was to be stupid all along, and as such, acting a fool achieves the goal - making their approach smart indeed. But, if someone wants to put out a fire and tries to do so by waving a fan at it, you may tell them that what they are doing is counterproductive.
If you have been hurt, and wish to communicate that to someone to bring about accountability, you can do this without insult too. You can point out the specific action they did, and express how it impacted you. If you tell them they are cruel, idiots, crazy, you can expect as much abuse to come back at you.