The way I see that was, firstly, to show where the dark had lead Anakin in comparison to Padme who is laying still in the light, at peace. To me, it shows where his dark deeds has lead him which is to torment and anguish beyond anguish.

I suppose the more in universe reason would be that Sidious couldn’t care less that Anakin was in pain. They are the Sith: they aren’t known to be particularly gentle or caring.

^ The person above me is fully correct, Sidous did not care about Anakins pain in fact it was one of his very first lessons in the Darkside. During no time in the surgery was he given any sort of relif or painkillers and this lasted for 8 hours.

As an addendum to what Infantry said: Sidious didn’t ask to the med droids to put anesthesia in Anakin on purpose. He wanted him awake in order to feel the pain during the whole procedure, since pain is part of the Sith philosophy of life. Vader being born out of pain was not only metaphorical, but also literal.

Yeah. Palpatine wanted him to be as angry as possible, and pain usually makes us angry.

My guess was always that the dark side fueled him.