Sometimes, but it's mostly ignorance. The only example I can think of related to intelligence itself may have been a certain german post-war trial that may earn me a ban for calling out by name.

Intelligence is definitely up there in the criteria we use in determining whether a group is ‘inferior’ to us or not. One of the main points vegetarians use when arguing against meat production is the proven intelligence of the animals being slaughtered. It’s easy to imagine a scenario where a disabled person has their autonomy limited or completely removed by the notion that they have less intelligence than others, especially for people with learning difficulties or speech differences. With human beings they’re removed of their intelligence so that they can then be stripped of their autonomy. “These people don’t know what’s good for them, but we do, because we’re the intelligent ones…” And once they’re designated unintelligible lumps of flesh then anything goes, because then they’re not seen as human anymore.

Depends. People who are immersed in a tribal worldview (us vs them) will use “Not one of us” to often justify cruelty or amoral behavior. People who see the world in legalistic terms (good people vs bad people) will use “bad people” to justify similar cruelty. Then people just start changing the definition of “bad person” ( to exclude themselves and those who agree with them, of course).

More generally, we dehumanize anyone not considered to be in our in-group. Assuming low intelligence may be part of that (the colonizers certainly believed that about the “savages” they were up against). We’re less likely to show empathy to anyone we consider to be in our out-group. Tribalism is a shitty trait in all of us.