When generating an evil world, I aim for a hostile environment, as opposed to hostile critters/beasts.
[WORLD_GEN]
[TITLE:EVIL WOODS]
[DIM:257:257]
[EMBARK_POINTS:5000]
[END_YEAR:1050]
[BEAST_END_YEAR:125:25]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:150:400:200:300]
[RAINFALL:25:100:400:400]
[TEMPERATURE:25:75:400:400]
[DRAINAGE:20:100:400:400]
[VOLCANISM:25:100:400:400]
[SAVAGERY:0:100:400:400]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[MINERAL_SCARCITY:200]
[MEGABEAST_CAP:75]
[SEMIMEGABEAST_CAP:150]
[TITAN_NUMBER:33]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:52]
[NIGHT_TROLL_NUMBER:0]
[BOGEYMAN_NUMBER:26]
[VAMPIRE_NUMBER:0]
[WEREBEAST_NUMBER:0]
[SECRET_NUMBER:52]
[REGIONAL_INTERACTION_NUMBER:52]
[DISTURBANCE_INTERACTION_NUMBER:52]
[EVIL_CLOUD_NUMBER:30]
[EVIL_RAIN_NUMBER:30]
[GOOD_SQ_COUNTS:50:500:1000]
[EVIL_SQ_COUNTS:200:2000:4000]
[PEAK_NUMBER_MIN:20]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:30]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:4128:13:12]
[REGION_COUNTS:MOUNTAINS:8256:9:9]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:1000]
[RIVER_MINS:50:50]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:90]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:40]
[CAVERN_LAYER_WATER_MIN:35]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:40]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:1500]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
I turn off vampirism, since they're easy to exploit; I also turn off werecreatures, since they're far too easy to deal with as a rule - and each time a werelizard turns up, it's one fewer Giant or Cyclops that could have spawned instead.
Personally, I find that evil woodland biomes are the toughest, as they tend to spawn more undead flyers than say, a tundra. <----not scientific; just anecdotal.