Try to avoid typical "evil" names because they are overused on Impmon. You wouldn't believe how many Darkness, DarKness, Evil, Beezlebub, Lucifer etc. named Impmon I've seen (and I'm not playing that long). However, if you want something dark or cool sounding, go for something cryptic: favourite villain in a show/anime (but not something extremely popular) or overused names in different languages (I really like Dunkelheit or Finsternis, both german words for darkness), it sounds cool and it is a fitting name while very few people use it. Or, like OmegaForte said, use your favourite anime character. Though, having an Impmon named Goku, Luffy, Naruto or even worse Ichigo (strawberry) is slightly... out of character for Impmon. I named mine after an anime character that reminded me of Tamers' Beezle.
GDMO status: rarely playing
Name: HikaruIzumi | Level: 48 | Server: Lilithmon
Digimon: lv 50 Gaomon (Husky) Starter | lv 46 Dracomon (Drake) 4/5 | lv 46 Monodramon (Slayer) 5/5 | lv 54 Gabumon (Wolfie) 4/5
lv 66 Impmon (Jolly) 5/5 | lv 46 Agumon (TRex) 4/5 | lv 46 DemiDevimon [Soulmon] (Reaper) 4/5 | lvl 46 Dorumon [dex] (Chaos) 5/5
lvl 46 Mechanorimon (Tempest) 4/5 | lvl 41 Demidevimon [Myotismon] (Thanatos) 5/5 | lvl 48 Renamon (Vixen) 5/5