If both benign and malignant tumours are formed by abnormal cell division, what makes malignant tumours cancerous and benign tumours non-cancerous?

Power to invade. Tumors are in fact caused by the accumulation of a series of genetic mutations. If something is going to be cancer, the mutations destabilize the genome and eventually mutations give the ability of the cells to invade and spread as they do early after conception. The idea that tumors are "cells dividing too rapidly" is fundamentally wrong and confusing -- it's the loss of controls.