What do you mean HAVE to be evil?
A mod which allows me to make my own custom world from scratch should have the option to choose so. Is it just a technical thing that can't be changed or something?
You can stop this. The reason you always get this with the default settings is that 'large' regions are actually very uncommon, and almost always end up being the ocean. If you set the evil or good 'large' region count to even 1 tile, it's almost always going to be an ocean. What I do is set the large region counts to zero, and then pump up the medium region counts to a much higher number. I feel this gives a much better balance and variety of regions.
EDIT: Yes, in the example screenshot you can see that your only large regions are the two oceans. Since the tile counts for large regions for good and evil are both above zero, you have exactly one good ocean and one evil ocean, and zero neutral oceans.