Evil seems on the whole bad but it’s kind of a mixed bag.
Wielding it increases the disease length, removing it lets the disease tick down. There’s a lesser variant where you’ve got the item, and a greater variant where you don’t have it (item unequipped). They use the same counter, though.
At low levels of the lesser evil disease, you gain a point of dex. However, that goes away and eventually starts to become a penalty to dex, along with penalties to int and per. However, strength goes up and keeps going up.
At all levels of the greater evil disease, you take hits to all stats (much worse than the hits you’d get under the lesser evil). The wiki seems to be right except that it claims small bonuses to int and per at low levels of the disease which I’m not seeing in the code. If someone could confirm that by briefly equipping an evil item, we can update the wiki.