Eating Foods that are Rancid is not dangerous and won't make you deathly sick but new molecules that form as oxidation occurs in the Rancid Food may lead to digestive issues.
But other than that the Rancid Food is perfectly safe to eat but you might have stomach aches.
Rancid Food is not as nutritious either because oxidation destroys the good fats and some of the vitamin content that is needed for optimal health.
But you're okay to eat Rancid Food if you want too but I personally wouldn't eat food that's gone Rancid but that's just me.