Pilots have access to virtually all critical systems on board; if they wanted to kill everyone on board, they could simply fly the plane into a mountain. Pressurization is mostly automatically managed, but if it fails, the pilots must be able to manually depressurize before landing.
Because if you have control of the aircraft you already can kill everyone if you wanted. Manual control of cabin pressure could be useful if the pressurization system malfunctioned and needed to be manually overridden.
All vehicles that can be steered have that possibility, even cars: If there is a steering wheel, it is possible to deliberately crash the vehicle. If it is possible to shut down engines, it can also be used for malicious purposes. In other words, there is always the possibility of using everything for malicious purposes.
On the other hand, the ability to shut down a system that has failed can save everyone on board.
There are approximately 40,000,000 flights each year. Almost all occur without incident. In the entire 120-year history of aviation, only a few accidents have resulted from the pilot intentionally crashing the plane.
