It's hard to make awesome-looking covers look very different from each other in enough aspects, when one character is a fire-breathing, metal-plated, dark dragon, and the other character is, well, Diablo. Both are very related to fire and warm colors (like red), both are very dark in color, both have very hard skin, etc. I do, however, think that it's unfortunate that there are so many similarities, despite the What did you expect? The two characters are always relatively alike in general graphical theme.