Chernobog is a name many authors use for their big bad guy. But was Chernobog really an evil god? Was he really Belobogs evil opposite, or is there some proof that he simply represented different aspects that didn't fit with Belobog?
Was Chernobog evil before Christian monks started to compare him to the Devil?
To avoid "good vs. evil" dilemma, let's just say that human sacrifice is bad, representing 7 major sins is bad, spreading diseases is bad, etc.
Killing enemies is not bad, guiding/harvesting souls of the dead (like Grim Reaper) is not inherently bad, giving someone unnatural powers is not bad(unless the cost is like slaughtering your loved ones or something), etc.