To make a map have alot of evil just make a custom one with custom parameters. Use one of the Large Region/Island or whatever you like. I just use pocket. Copy the map you want, rename it ("t"), and press "d" (I think) to change the parameters. All of it probably won't make sense if you never used this. What you want to look for is something like "Number of Evil Small Subregions", "Number of Evil Medium Subregions, "Number of Large Evil Subregions". Play around with that by making those numbers bigger. You need to find the right balance that will let your world be covered with evil but at the same time let other civilizations exist. Humans, Dwarfs, and Elves can't exist if all Hills+Grasslands, Mountains, and Forest are evil.
Another thing you'll want to do is increase the number of titans. I set it to 1000 and set all titan attack requirements to None so they come whenever they feel like it. Increase the number of Mega and Semi Beasts too. Increase the number of natural caves (mountain and non mountain) so that they have lairs in your embark sites. I don't touch the number of demon types and night monster types. For some reason that lags up my comp. Set "Reveal all Caves" (or is it "Reveal all Lairs") to yes if you want to see exactly where these monsters live on the embark map.
Have fun with a Titan sitting right next to your wagon the moment you load the game.