Here's one to think about.
When Anakin killed Dooku he was coerced into doing it by Palpatine (even though not so deep down he probably wanted to do it anyway as revenge for losing his arm)
When Palpatine fought Mace he was scarred and changed by Vaapad and the force lightning Mace turned back on him.
Fighting Yoda would have taken it out of him too (for my money that's the real big battle in ROTS. While Ben and Ani fight over love, Ani's soul and a fractured friendship gone bad, Yoda and Palps are the true embodiment of Light vs Dark, Jedi vs Sith...Empire vs Rebellion in a way).
But, when it comes down to it the best thing, from the Emperors point of view, was Anakin being burned in the lava. A full-powered Darth Vader would have eventually killed Palps, and he knew that. So did Anakin - he said it to Padme on Mustafar and Luke on Bespin.
But Ben hacking Vader to pieces broke him body and spirit. Putting him in the armour trapped him. Telling him of Padme's death, by his own hand, gave him nowhere else to go. The only people who could have helped him had been slain by his own hand. Less than he was but still powerful, Vader was the perfect tool for Palpatine. He had a titan that he could control. Without the dip in the lava he could have controlled Anakin for so long, but only for so long.
So, my point is - did Kenobi do Palpatine a VERY big favour defeating Vader in the manner he did?