1: Anger issues
There are those who, for whatever reason, just do destructive things for the sake of their destructiveness. They may be malicious narcissists, psychopaths, or just so self-centered in their impression that the whole world is against them that they will blindly lash out at anyone and everyone when they get the chance. They might think they're misunderstood and want to communicate with the world by harming it in some way
2: Do it for the Fun
Some still do it for the "fun" of destruction. They may get a thrill out of reading news items about their work causing people trouble, or they may just take a fire-and-forget approach, creating destructive, self-replicating programs for the joy of it without much caring whether they ever see the consequences themselves.
3: Espionage
I'm not talking about sabotage here; I'll address that later. By "espionage," I mean attempts to gather information through underhanded means for reasons other than identity fraud and other directly, criminally profitable purposes.
5: The hacker instinct
Keep in mind the difference between a hacker and a security cracker. People with a hacker mindset usually find themselves eventually drawn to specific fields of interest. In some cases, that interest might revolve around understanding self-replicating mobile malicious code.
6: Romance and drama
Because of the character of certain online communities, it can be much easier sometimes to feed one's own delusions of the romance and drama of being a "Computer Hacker" for a longer time than in most other criminal enterprises where the physically gritty, and petty, reality of what they do becomes quickly inescapable.
How are computer viruses spread? 1. Email attachments.
The world became familiar with the phenomena of email attachments carrying viruses thanks to Microsoft’s Outlook Express automatically opening every attachment to every email you received a few years