Generally the term hacker is used in a negative connotation. It's normally used to describe a person who steals or destroys digital information from other people. However there are quite a few capable hackers who use their abilities for good. One example would be the Trace Labs organisation. They created a "game" where hackers can earn points by finding information related to missing people which can then be redeemed for rewards. In a sense this motivates people to work together and solve what would have originally been an almost impossible task.
There are also hackers who hack into systems to bring awareness to the security flaws. If these flaws were to be discovered by a "bad" hacker, it could lead to a huge wave of panic caused by potentially stolen data. They could also insert a virus into the system. Check out chapter 81 if you want to learn a bit more about viruses. Depending on the skill level of the programmer, a virus can do basically anything that is possible in a digital environment from locking/deleting data to getting you mom's credit card information.
Join the discord server to receive updates on new chapters and talk about books:
https://discord.gg/jGCnyZ