"Count's Revenge" is J.H. Walsh's adaptive text influenced by Alexandre Dumas' The Count of Monte Cristo. Given the Romantic nature of the original text, Walsh carried these characteristics over into his own. Romantics, especially those who followed the Gothic path, believed that all people possess an unknown mixture of good and evil within themselves (this means that all people are capable of good and evil).
As for Walsh's text, when referring to the hero as possessing "a touch of the devil" refers to the idea that, although mostly good, the character does have the ability to be a little bad or evil. As the universal motto (eye for an eye) acknowledges, many people are willing to do things they would not normally do when faced with revenge. Essentially, the necessity of revenge brings out the evil in the Count.
I have linked the "In-depth Analysis" of the original text from the eNotes page.
Thank you literaturenerd.
"A touch of devil" would hint that there are parts of him that are evil and bitter. The character is built around vengeance, so it can be assumed that the "devil" in him is the part which seeks revenge.
Thank you missmyers.