Having a tooth cavity itself won't kill you and you can get the tooth cavity filled in if you don't wait too long.
What can kill you though is when you leave the tooth cavity untreated and then that cavity leads to a tooth infection and then gum disease.
That tooth infection and gum disease can then infect your blood and can cause death eventually.
So while death can occur from tooth infections it's very rare that a person dies from a tooth infection even when left untreated.
But still if you have a tooth infection it's best to get it treated so that it doesn't potentially lead to other health problems and also cause your other teeth to rot out.
You want to keep your adult teeth because it's either adult teeth or dentures or dental implants or eating baby food.