Well it's not the Jedi way to kill an defenseless life form. That is what a Sith would do.
So killing Anakin in that manner in cold blood would be akin to giving in the dark side, if only for a moment, which would be bad. Case in point pretty sure if Anakin had died the Rebels would have lost on Endor and Sidious would have ruled for like ever...
Kenobi: Darth Vader is dead, how'd your fight with Palpatine?
Yoda: Failed have I...
Kenobi: What the **** Yoda, I just killed my brother, the kid who I raised and you couldn't kill a wrinkly Chancellor?
Yoda: Most powerful Sith in history he...
Kenobi: Don't feed me that tier bullshit! What happened to "Powerful enough to face the Emperor you are not", apparently neither are you.
Yoda: Gone the Order may be, but Grand Master I still am!
Kenobi: Exactly so rebuild your lightsaber, train some more, and then go kick his ass.
Yoda: Right you are!
*Yoda falls to the darkside, and pwns Palpatine on neutral ground, kills him, becomes the new Jedi Emperor.
Registered: Mar 2014
Location: The Misty Mountains
A large amount of Jedi survived order 66 including but not limited to Kenobi, Yoda, Hett, Dark Woman, Sunstar, Ferus, Jax, Lanarth, Ti, Paratus, Kota, Kanan, Kento and many more. If they worked together instead of all hiding they could have mustered a great resistance against the Empire