Sometimes leaders make firing desicion-if there is an evidence of bad performance should the leader investigate reasons of it before hiring? Why?

One should investigate as well as give fair opportunity to provide explanation, one should not take unilateral decisions without providing with an opportunity. 

Definitely! Leaders should investigate the cause of misconduct or whatever caused the action. Sometimes it is usually a misunderstanding on behalf of the employee and we all deserve second chances. Bad performance can be reprimanded by offering a refresher training course or just a written warning. In the event that the bad performance has continued in spite of all warnings then leader can take further action by terminating employee. Another cause of bad performance is as simple as bad management. 

Of course they should. Performance could be affected by other reasons more than by the employees. Even if the employee shows poor performance, the organizations should try to support, educate & train, facilitate, and contribute to the performance enhancement process. The company may loose a valuable opportunity by firing any of its employees. Such a decision should not be made untill after a very good consideration. Some companies rotate thier poor perfomers within other positions untill they find the best fit. This could be good recommendation if the rotation does not exceed3 rotations,