Why Did People Write Viruses In The 80s & 90s?

Why did people write malware in the pre-internet days? Back then, there was no way to make money by writing malware. So why write them in the first place? The lack of a financial motivation meant that virus authors had a plethora of other motives - and this diverse mix of motives had, as we shall hear, an interesting effect on the design and style of viruses created at that period.