Why do some programmers develop malware/ viruses and what benefit do they get from it?

Malware isn’t about messing with you, joking around, or just causing damage — it’s all about profit.

To understand why all this malware is out there and why people are making it, all you have to keep in mind is the profit motive. Criminals make malware and other nasty software to make money.

If you used computers in the 90s, you remember the first mainstream computer viruses. They were often practical jokes of just proofs of concepts, created to mess with your computer and cause damage by people with too much time on their hands. Getting infected by a piece of malware meant that your desktop might be taken over by a pop-up proudly proclaiming that you’ve been infected. Your computer’s performance might deteriorate as a worm tried to send as many copies of itself out onto the Internet as possible. A particularly vicious piece of malware might try to delete everything from your hard drive and make your computer unbootable until you reinstalled Windows.

For example, the Happy99 worm, considered the first virus to spread itself via email, existed only to spread itself. It emailed itself to other computers, caused errors on your computer while doing so, and displayed a “Happy New Year 1999 !!” window with fireworks. This worm didn’t do anything beyond spreading itself.

Malware creators are almost purely motivated by profit these days. Malware doesn’t want to inform you that you’ve been compromised, degrade your system performance, or damage your system. Why would a piece of malware want to destroy your software and force you to reinstall Windows? That would only be inconveniencing you and the malware’s creator would have one less infected computer.

Instead, the malware wants to infect your system and hide quietly in the background. Often, malware will function as a 

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keylogger and intercept your credit card numbers, online banking passwords, and other sensitive personal data when you type it into your computer. The malware will send this data back to its creator. The malware’s creator may not even use these stolen credit card numbers and other personal information. Instead, they may sell it cheaply on a virtual black market to someone else who will take the risk of using the stolen data.