Why people create malware's? What's the reward? Just to screw with others?

It is more interesting and challenging

Malware is why I'm a dev, I started as a technician and spent so much time cleaning up malware that I wanted to know how it got there. I started researching stuff and ended up working as a dev, namely web, but some small software projects here and there.

because they get paid for creating them or selling the outcome of using them (botnets).

When creating stuff doesn't make you feel god enough and you start manipulating stuff

Malware can be useful ;)