You're not remembering the
scene correctly.
1. The Jedi come to ARREST Palpatine.
For what crime?
You can't just arrest people for
being Sith. You have to explain what law he broke.
What law was that, Zack?
2. Palpatine says "you can't arrest me, I'm a dictator"
That's not what he said, he asked Windu if he was threatening him. Windu said the Senate would decide his fate. He responded by saying he was the head of the Senate. (And by the way, the
courts should decide his fate. Prove he committed a crime in court.) He then pointed out they were simply committing treason, which is a crime that has a punishment of execution in our modern and civilized society.
What, exactly, is the charge? That he's a Sith?
So what?
Is there a law against not believing the Jedi philosophy of taking children away from their parents and teaching them from the age of a toddler how to fight with a deadly weapon and avoid procreating?
That's just WEIRD.
3. Palpatine ATTACKS FIRST, killing three Jedi in like five seconds.
They pulled out lethal weapons first, and were not arresting him for any actual crime. What were the charges? No charges were ever listed.
4. Windu defends himself.
Uh uh. Palpatine defends himself against their unprovoked threats.
5. Mace Windu is just standing there, when Palpatine suddenly attacks him with lightning
The dude just kicked away his lightsaber, and was threatening Palpatine with a weapon that can kill you pretty much instantly.
Palpatine continues to defend himself.
Again, what were the charges? What crime? What sin is simply BEING A SITH? No reason just "We're Jedi, here are our lightsabers, might makes right, you being a Sith means we can just lock you up. (What evidence? What warrant? Just the testimony of one Anakin Skywalker, a Jedi you already don't even trust, who goes on to KILL YOUNGLINGS? Yeah, he's a really reliable witness. Your accuser is a mass murderer!)
What's shameful are the Jedi and their blatantly illegal actions.
They got what they deserved.