Bug bite on leg was itchy, grew and is now a round purple bruise on my leg - assuming it is a spider bite but do I need to treat or will it go away?

Get checked. May be just as you suspect, but maybe not. Best to get it checked by your MD or Dermatologist. Good Luck.
See your PCP. Most bites are relatively harmless, leaving just an itchy patch of skin behind. However, some bites can carry disease. Lyme disease is a good example.