I am going to be graphic so if you are squeamish just don't keep reading - I will tell you that my vet assures me that spay/abort is minimal suffering to the kittens. At the shelter and clinics, it's nigh unto impossible to get a vein in a kitten so they are jabbed into the heart or lungs - I attend all my euthanasia cases so I know first hand what goes on. The most humane according to several vets that I have asked, is blunt force head trauma but here in CA, they can only do that to some livestock. With spay/abort, the kittens go under the anesthesia and don't wake up and are also given a shot just to ensure that the little spirits "have flown".
