star wars - Why was Windu going to kill Sidious?

Killing wasn't a Jedi way, no matter how much big the threat is. Sidious was unarmed and weak, too. Windu could have arrested Sidious easily. But, still he didn't care. He even ignored Anakin's words. Why? He said that a Sith was too dangerous to be kept alive. But, still killing wasn't a Jedi way and proper way of doing this was to discuss this with Jedi council first.

Assuming Windu was a Jedi without darkness, why did he choose this way?

Answer

There were 3 reasons:

• 
  

  First, Windu was NOT initially trying to kill Palpatine but to arrest him

  
  


• 
  

  Then, Windu saw a grave danger as Anakin - whom he already didn't trust around the Sith earlier - was seemingly teetering on being convinced by Palpatine to join him against Windu

  
  


• 
  

  At which point Windu realized that killing Palpatine is the only feasible option - either Anakin will join him, or even he gets arrested, he will win in courts etc..

  
  

More specifically, according to the script, the following dialog took place (block quotes from the script):

---

1. 
   

   First, Windu was NOT initially trying to kill Palpatine but to arrest him:

   
   

   They stop as MACE forces PALPATINE to drop his sword. 
   PALPATINE and MACE start yelling at each other. 
   
   MACE WINDU: You are under arrest, My Lord.
   
   



2. 
   

   Then, Windu saw a grave danger as Anakin - whom he already didn't trust around the Sith earlier - was seemingly tethering on being convinced by Palpatine to join him against Windu:

   
   

   PALPATINE: Anakin! I told you it would come to this. I was right. 
   The Jedi are taking over.
   
   MACE WINDU: You old fool. The oppression of the Sith will never return. 
   Your plot to regain control of the Republic is over . . . you have lost . . .
   
   PALPATINE: No! No! You will die!
   
   
   PALPATINE raises his hands, and lightning bolts shoot out. 
   They are blocked by MACE's lightsaber. 
   PALPATINE is pushed back against the window sill.
   
   PALPATINE: He is a traitor, Anakin.
   
   MACE WINDU: He's the traitor. Stop him!
   
   PALPATINE: Come to your senses, boy. The Jedi are in revolt. 
   
   They will betray you, just as they betrayed me.
   
   PALPATINE: You are not one of them, Anakin. Don't let him kill me.
   
   PALPATINE: I am your pathway to power. 
   I have the power to save the one you love. You must choose. You must stop him.
   
   MACE WINDU: Don't listen to him, Anakin.
   



3. 
   
   

   At which point Windu realized that killing Palpatine is the only feasible option - either Anakin will join him, or even he gets arrested, he will win in courts etc..

   
   

   MACE WINDU: You Sith disease. I am going to end this once and for all.
   
   ANAKIN: You can't kill him, Master. He must stand trial.
   
   MACE WINDU: He has too much control of the Senate and the Courts. 
   He is too dangerous to be kept alive.