If untreated cancer. Most scrotal swellings are benign. You will not die from testis torsion or epidymitis, but treatment will make you feel better. Non-tender swellings are also benign unless the testicle is enlarged and hard, or has a hard lump or nodule on it. This could be cancer. Most testis cancers are cureable as with lance armstrong, but not if ignored and allowed to spread all over your body. See a doctor.
Depends. Scrital swelling has multiple causes. Failing to find out what's wrong can put you in grave danger. See a doctor asap to get the cause diagnosed.
Its possible. It really depends on the cause of the swelling. If the underlying problem were cancer, than ignoring it would lead to tumor growth and significant risk of a fatal outcome. If the problem was due to a hydrocele, the risk of ignoring it would be less dire. However, there is no good argument for ignoring a swollen scrotum. See a doctor to diagnose the problem and get whatever care you need now!