Any tattoo design representing opposing elements, words or images are considered to have a "good vs. evil" message. This message is best designed in one of two ways -- either both elements are blended together to form one image with equal parts, or they are represented separately on opposing parts of your body, such as shoulders, feet, calves and others. In addition to the design and placement, you must also consider colour scheme and style.
An ambigram is a word that reads as one word, one way and a completely different word when flipped or looked at in a different way. Find an online ambigram generator or design your own, using two words that represent good and evil. The two words should have the same amount of letters to make the design a little easier to develop, and if using an online generator, realise some words simply can not be made into an ambigram. Some word pair ideas are, "good, evil," "smile, frown" and "angel, devil."
Angel and Devil
Angel and devil caricatures are anything from graphic and detailed representations of each to more reserved and charming images like cartoon characters wearing angel and devil costumes. The tops of your shoulders are a classic spot for these tattoos, representing the angel and devil whispering in your ears. If you don't want large, detailed tattoos on your shoulders, consider a simple item, depicting the same message, such as a halo and a pitchfork, or wings and horns.
"Star Wars" Yin Yang
A yin yang is a classic design of opposing elements, generally sold black and white forms, moulding together to form a perfect circle. They are meant to portray each elements reliance on the other for completion. A "Star Wars" yin yang is the classic design filled with images of characters from the movie, such as Darth Vader or Emperor Palpatine on one side and Yoda, Luke Skywalker or Han Solo on the other. To make the design crisper and more lifelike, consider doing each character in a detailed portrait style
Goat and Lamb Head
Goats represent evil in some cultures, as the biblical Satan is described as adorned with hooves and horns like a ram, while the softer and more submissive nature of a lamb symbolises the good and beauty in people. Consider tattooing a goat head on one arm and a lamb head on the other arm but in the same location. Other viable spots are opposing shoulder blades, feet, calves or thighs.