Sure, it's to show he's evil, but it's not like Sidious is an animal or anything, it seems that the very least of the action was to inspire fear into their hearts or something.
My personal feelings as a fan interpret Sidious's roar of rage to be him finally unleashing his aggression and his darkness out. The man's been locked in a facade of peace and tranquility for decades, and now is his chance to kick ass. It does seem to make him somewhat animalistic - an ironic contradiction to his regular level of sophistication - and to let the viewers know:
"This guy is the ultimate Star Wars villain. The ultimate evil."
There is no in-universe explanation for it. Considering how these Jedi have thrown themselves into battle before, at the risk of death, they don't strike me as the type to get scared easily.
That roar was surprising though. I agree with Darthsith, I believe he was using Force Rage along with his Jedi-hating anger. In addition, I think he was also using Master Speed with his jump otherwise there is no reason why Kolar couldn't block Sidious's stab.
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BlackZero30x created this a-'Maize'-ing signature! =)
Mace was actually moving pretty slow, Agen and Saesee displayed worse reflexes than Mace or Kit did, Kit moves quickly, the quickest of all four jedi in my eyes, just look at how he pulls of this real quick spin.
Finally someone who agrees with me!!!
What has Kit done? Won a battle at Mon Calamari due to the fact that the battle took palce underwater, got defeated by Asajj in a close one in The Cestus Deception and got pwnd by Sidious in 7 seconds even though he was fighting besides perhaps the best Jedi dueler in the order.
Prior to their duel, Asajj unleashed a barrage of mercenaries on Kit (and Obi-Wan) in order to STUDY how he fought, and hence work out his weakness. Not only was she able to gain much advantage by just watching how Kit fought, but her mercenaries would have likely tired Kit out, thus creating more of an advantage for the dark Jedi.
If you’ve read the Cestus Deception, you would also know that apparently, Asajj came within a hairbreadth of defeating Mace Windu.
Qui-Gon would have beaten TCD Asajj, even Obsession Asajj he'd give a run for her money, and he's have lasted at least twice as long as Kit did against Sidious.
And how do you know this?
Qui-Gon lacks speed and dexterity because of his old age. In the Phantom Menace novel, it also mentions Qui-Gon’s strength diminishing because of his old age.
I also believe that Shaak Ti once stated that Kit Fisto, Mace Windu, Agen Kolar, and Saesee Tiin were the BEST Jedi in the order (probably with the inclusion of Yoda). Qui-Gon Jiin may have been very powerful in his prime, but I doubt he would have been able to last any longer than the other Jedi in the room against Sidious during the time of TPM.
Medvock, Kit was certainly more skilled or - at least - better prepared than Tinn or Kolar, but he still lost. Mace and Sidious are quite a bit above him in power.