You can leave butter out at room temperature for a few days or so.
However refrigerating the butter helps prevent bacteria from growing in the butter and extends the life of the butter.
Butter is mostly safe at room temperature and as long as you use the butter within a few days or so then the butter is safe to leave out.
It's safe to leave butter out on the counter but only for 1 to 2 days.
Many people leave butter on the counter for a day or two to soften the butter and that is generally okay as long as you keep the butter covered to prevent flies and gnats etc from getting to the butter.
After a few days though the butter can go bad and rancid and should be thrown out.
It's best to keep butter refrigerated though to prevent the potential of bacteria from growing on the butter.
Yes eating rancid butter can and will make you sick.
If you eat just a little rancid butter then you may just get vomiting, diarrhea, stomach cramps etc.
But if you eat too much rancid butter then you may get food poisoning and in rare cases botulism and die.
It's not worth the risk to your health to eat rancid butter or other rancid food.
You can tell when butter has gone bad by smelling it and looking at it.
When butter goes bad the butter will smell rancid and can have mold growth on it and also may change color or become hard.
If the butter has mold, smells rancid or looks bad then throw it out and don't risk getting sick.
Butter can eventually go bad in the fridge.
Butter will keep in the fridge for for up to 3 months as long as you keep the butter covered in a sealed container.
When butter is left out at room temperature it can go bad within a few days so always keep the butter refrigerated when not using it.
If the butter smells rancid has mold or has changed color then it's bad and should be thrown out.
When butter goes bad the spoiled butter will smell rancid.
If you smell the butter and it smells rancid and makes you want to vomit then throw it out because it's bad.
The spoiled butter may also have mold or the color or texture of the butter can change and if so throw it out.
Butter should last for a few days at room temperature and when refrigerated butter can last from 1 to 3 months.
When leaving butter out at room temperature to soften it's usually safe to leave your butter out for up to 2 days.
If you leave your butter out longer than a few days then you risk it getting spoiled and growing bacteria.