Least evil way to make soap? I'm not a chemist but something of a tinkerer and adult boy scout.  Currently if you want to make soap, you need fat and lye.  Fat I'm good with, it's easy to get and you can even get it from plants. Lye usually requires wood ash though and that I would rather avoid, if possible. If not possible and burning something is mandatory, then I want the biggest bang for my buck. That means I want to get the most lye I possibly can with the least pollutant release, especially greenhouse gasses and we shouldn't be thinking of just wood. Anything that will get us some low impact soap in the end. We may even want to replace soap as an idea with something else that you can make at home and wash humans, clothing and dishes with.
By far the largest source of lye nowadays is the electrolysis of concentrated solutions of $\ce{NaCl}$ or $\ce{KCl}$ in water, the famous and incredibly industrially important chloralkali process, producing millions of tons of $\ce{NaOH}$ and $\ce{KOH}$ every year. While electrolysis is an energy-intensive process, presumably the electricity could be supplied by a low-carbon energy source such as solar or wind power. 

"Replacing soap", in the sense of halting the usage of surfactants, is not a good idea and likely not even possible. Surfactants are too useful a class of substances. In fact, soaps are likely the most environmentally friendly surfactants, given that they're based on a simple hydrolysis and acid-base reaction with common biomolecules, fatty acids and triglycerides. Other surfactants such as alkylbenzenesulphonates (dishwashing liquid), tetraalkylammonium salts (hair conditioner) and perfluoroalcoxylates, among others, can be toxic to aquatic lifeforms and take longer to degrade in the environment. The excessive usage and improper disposal of surfactants is a recognized environmental problem, but tackling soaps won't really get you far in solving the issue.