If you look at some of the popular angel tattoos, you would see a resemblance towards the devil tattoo. The angel would either have an evil face or dark colors not accurately depicting an angel. The angel and devil could be fighting while the devil would be winning. Tattoo designs can depict stories or symbols without other people really knowing the truth behind the design. The design could 'look good' to a person but the real meaning of it is only known by the artist itself.
Many tattoo artists are designing angel tattoos that can resemble both good and bad, depending on the person's personal conviction and style. By doing a little search on angel tattoos you could find many devil designs mixed in the results. The reason is because in today's world there is not a fine line of angel and devil tattoos. The good and bad between the two designs is whatever the person wishes it to be. This is not to say that all angel tattoos are leaning towards the devilish side, as there are many angel tattoos that depict what you might see in a book about angels, beings of light and fighting for good.