There is no form of death that would be acceptable or considered humane for those against the death penalty, and the death penalty as applied in California is not a deterrent against violent crime.
The threat of a death sentence should make any person think twice before committing a violent crime. However, knowing you're practically guaranteed 12-plus years of life, accompanied by the very strong possibility the death sentence will not even be carried out, tends to eliminate the threat.
If you eliminate the threat of a death penalty, you place the people assigned to provide care, custody and control of those violent offenders, in very serious danger.
The only way the death penalty can be an effective deterrent would be to apply it in a timely manner, relative to the violent act and conviction. The lengthy appellate process must be reworked and time limits put into place, so executions can be conducted in a timely, less costly manner.