What is the mortality rate if I don't get treated for meningitis?

High. If you truly have meningitis and do not receive the proper antibiotics, the mortality is very high. Refusing treatment is not really a viable option.
Depends. There are different types of meningitis such as viral and bacterial. Most bacterial meningitides carry a very high mortality rate (cannot quote a number due to different bacteria causing different rate) but very high if you get treated even, but if you are untreated it will definitely be 100%. Viral meningites, on the other hand, carry a better prognosis, but again depending on the virus type.