Eating expired peanut butter may not be as tasty as fresh peanut butter but it won't make you sick to eat old peanut butter.
I've eaten peanut butter that was past it's expiration date even after being opened and it's still good to eat and I've never gotten sick from eating the old peanut butter.
If the peanut butter is over 5 years old then it may taste even nastier but it still is very unlikely to make you sick from eating it but you might get a stomach ache but nothing serious.