I think an evil protagonist can be well done in RPGs, but it's often not. Usually you just have the option of 'pet the kitten' or 'kick it to death' ala something like the simplistic morality systems of a Bioware RPG. Games that do this well often don't gamify morality (no good or evil points). The Pathfinder video games for example, with the first you can be a harsh ruler of your barony, trying to make an 'orderly' realm. The second game even better, you can slip into dark powers, like Lichdom, as a way to battle the existential danger of the demon threat, which the games dialogue options support until you make that last choice over the edge. Tyranny is another RPG that executes evil characters that don't feel too mustache twirling.
I personally found the justification for siding with Minthara and the Goblins kind of thin in EA. I guess the Dark Urge kind of solves some of this (why do you want to massacre refugees? Because you have an urge to kill), but I'm hoping there's more emphasis on *why* beyond just murder for murder's sake. Although there does seem to be a good amount of reason for evil/mercenary Tavs to side against them (Lolth-sworn Tav might see punishing Minthara and Nere as traitors to Lolth more important), so maybe this isn't such a bad idea so long as you have the dialogue options to express you didn't do it to 'save the day.'
I think something that all those games have in common is that they are all to some degree or another, very much ABOUT being able to be evil. With the exception of Kingmaker, being able to play a properly evil character is very important to the experience of the game. In most rpgs you do have a central story that you have to get to. And there's only so much flexibility you can give in interacting with that story until the story itself falls apart. And those stories typically are made under the assumption that the player is going to be pursuing a good goal. Saving things, stopping another evil, etc. So evil paths are included out of obligation as much as anything else. Often really evil, selfish people wouldn't have much motivation to pursue the story in the way the devs have created it. And you can say "well the devs should do more to accomodate evil choices and make them complex and satisfying" but the thing is, that's not the story they want to tell or are trying to tell. And more often than not trying to accomodate more evil would lead to both sides being diluted.
Now where Tyranny and WotR differ is that providing deep evil paths were, from the beginning, a design goal as important as making quality good paths. The evil paths are part of the story that require a lot of focus and effort. And even in Kingmaker, the story there lends itself to evil very well - after all at the core of it, you're trying to attain power and authority. The constraints of the story there are that you always have to be working to keep control of your barony, which isn't something that will conflict with a lot of evil characters, so evil choices are easier to make work. So my thesis is that unless the point of the game is tied in with being evil, then the evil path just istn't going to be as fleshed out in most games.
