What is the ruling if a person kills someone who unfairly dismissed him from work?

If your manager unjustly fires you from your job, then he is an oppressor, and it was incumbent upon him to verify the matter. It is your right to appeal to a higher authority. However, killing him is impermissible, for it entails loss in both religious and worldly matters. Allah Almighty says: "And whoever kills a believer intentionally - his recompense is Hell, wherein he will abide eternally, and Allah has become angry with him and has cursed him and has prepared for him a great punishment." (An-Nisa 4:93). Furthermore, taking a life that Allah has forbidden is among the seven destructive sins. So, avoid Satan and his whispers.