In my experience, endowing any character (whether they be good or evil) with redeeming qualities will enhance their accessibility. Highlighting how human a character is will serve this end. In other words, the characteristics you give your characters can determine how good or how evil or how likable or how not likable each of these characters are.
In the present-day, in the time that has lapsed since thinkers like Kant, Nietzsche, and Sartre were authoring their most-highly recognized works, morality has been increasingly subjective. In the line of thinking engaged by such intellectuals, it has been encouraged that people conceive of their own morality and live by that morality.
That said, think of your audience. What will they identify with? For example, if your two characters (i.e. the one that is evil and the one that is not), engaged in a comparable experience, and if they behave differently under these circumstances, will what they do be engaged by the audience in a way that is consistent with your audience's own line of thinking about morality. This a tricky ground to navigate, as morality tends to be relative.
For example, perhaps we should think of a drug dealer in any kind of narrative. Imagine following their immediate course of action in their mode of employment from the most immediate activity they engage in, which is selling drugs on the street, for example. Then follow them beyond that, portraying both their motivation for selling drugs and the human consequences of their illicit endeavor.
Say that this drug dealer sells drugs to an incorrigible addict, who uses the drugs to great detriment. Perhaps, their life falls apart as a result of their deviant behavior. One might readily consider this a proximate cause of the drug dealer's so-called professional engagements. However, then engage the drug dealer's motivation for selling drugs. Perhaps, after he obtains the money from his client drug-user, he goes to the grocery store to purchase food that he then goes home to feed his impoverished family with. When he arrives home, his infant child is wailing out in a way that suggests starvation. This is where the human quality that audiences may or may not identify with comes in.
In this scenario, this character is selling illegal substances to feed his disenfranchised family. He is just trying to raise his children in an effective and basic way. However, he is doing so by suspect means. From this, the audience may gather that, while he is ostensibly evil via his drug-dealing business, he is just trying to serve an end that is commonly recognized as good. The perceived evil is located within his facilitating the drug user's downfall. By contrast, the perceived good is his service to this family.
Try to engage the characters in the aforementioned fashion. Consider the means and the ends they are serving through whatever they do. While the means may be suspect (morally speaking), their ends could be legitimate and widely recognized as positive contributions to society. Positive contributions to society, within the context of so-called evil activities, may complicate how your audience understands your characters.
I loved your question. I hope this answer helps. I'd love to edit your story as it unfolds.
