The problem with "stupid"
There is a movement in online activism to get people to stop using the word "stupid," because it is ableist. Some people do not understand why, so I will attempt an explanation. If you only want to know some alternatives for the word, you can skip to the end.
First of all, ableism is discrimination based on someone's disabilities, or lack of abilities. It is a perception that their disabilities make them a lesser person, even though this is based on something that has nothing to do with any of their personal choices. In the case of "stupid," it is a judgment based on someone's perceived lack of intelligence. They do not deserve to be treated the same way as other people because of this lack of intelligence.
I'm choosing to focus on "stupid" because it has several problems that go beyond ableism.
When people describe a person as being stupid, there are several reasons they might say this:
ignorance
misunderstanding, or a lack of understanding
not doing what the speaker wanted them to do, or thought was best for them
one of that person's decisions turns out badly
Several of these things have nothing to do with intelligence, most notably ignorance. No matter how much something can seem like common knowledge, everyone has a different background where they might not have had the chance to learn everything that you know.
The very idea of measuring intelligence is something that has problems of its own. In its original form, it was meant to compare children's school performance against their peers. Even if this were the only reason we used the test today, we are using it to divert the top performers and bottom performers away from the same classes as the other children. We now know that this is something of a self-fulfilling prophecy: any student chosen at random will achieve more in a setting with more challenging work and more attention from teachers. Similarly, students who get placed into special education do not do as well as they used to, not least because they have lowered expectations.
Modern psychology has established that human decision-making skills are based on biases and heuristics, rather than logic. Logic is something that has to be learned and consciously practiced. We can tell ourselves we are being logical and rational, but sometimes we are just privileging our own position or emotions over someone else's. Someone who says "I'm just being logical" is usually just an asshole, whether or not their logic is soudn.
Second-guessing someone else's decisions is another way that we like to make ourselves feel smarter than other people. One of the manifestations of this is the "Monday morning quarterback," where people talk about what the team should have done in last night's football game. We have the advantage of seeing the consequences of the teams decisions, having more knowledge of the situation in general, and having the time to think about the merits of all the options. Some of us, of course, do this with ourselves all the time, and might even call ourselves stupid. (Every day... all day long...)
Finally, whatever it is that we call intelligence, there are several different kinds of it in people. I took an IQ test a couple of years ago, for the first time in my life, because I was trying to find out if I had ADHD. (The psychologist's answer at the time was, "I think you're just depressed," but that is another story.) They ranked me on four different categories, with scores that were as low as 117, and as high as 141, with a 95% confidence interval. One standard deviation from the norm is 15 points, which means there are almost two standard deviations between those scores.
What I am trying to say is that sometimes I can look very smart, and sometimes look relatively dumb. Or, y'know, dumb enough to make you wonder how it is that I looked so smart a few minutes ago.
So, who are you to call anyone stupid? If you think you can judge whether someone is unintelligent, you're really just a jerk. And if you think that we could make the world better if we "kill all the stupid people," that's called eugenics. It's one of Hitler's ideas that history has not looked favorably on. I prefer not to associate with people who think our world needs more genocide, and there are many other people who feel the same way.
Now this is the part where we look at alternatives to "stupid." First of all, searching for "alternatives to stupid" can get you some decent results. But here are a few that I try to tend toward:
illogical
nonsensical
ignorant
reckless
negligent
If you are thinking of using the word "stupid," try to ask yourself, "What is it about this that I think is stupid?" Then say that answer instead.