Usually, a benign tumour does not turn into a malignant tumour. However, a large benign tumour can, because of its size and location, cause harm. If this tumour is located near the tubules serving your kidneys, they may cause blockage - definitely not a good thing.
"Benign" - if you do opt for surgery, insist on a biopsy. This is the only definitive way to determine whether the tissue is malignant or benign, by putting the tissue under a microscope and looking at its cellular structure.
If you're interested in learning about how cells switch from being "normal" to being cancerous, look up "oncogenes". As you can guess from the word, the problem starts at the genetic level - mutation, transcription errors, replication errors, etc. It's a very interesting area in cancer and molecular biology research.