dtgreene: This isn't quite as bad as my example, provided a couple things are true:
1. There is a choice to be made, and both choices are viable (as in, the game doesn't get unreasonably difficult if you choose to not keep slaves).
2. The game doesn't encourage you to keep slaves; as in, the game doesn't present keeping slaves as the good and noble thing to do, and doesn't keep pressuring you to keep them.
babark: I don't think your criteria really work. Often, doing the right thing is hard, and many games reflect that.
(spoilers for Deus Ex: Human Revolution and Spec Ops: The Line ahead)
In Bioshock, theoretically, killing the innocent little girls provides you with more raw material to upgrade your abilities than saving them. So you could do the nice thing and not kill them, but then you'd be weaker.
In Dishonored, everyone calls you an assassin and you are celebrated for it. One of your keybindings (the left mouse button) is attached to a brutal dagger stab. You can level up and get all these cool powers that involve brutal ways to kill people, including nonviolent characters. So playing a pacifist run or an unseen run is much harder, and much more satisfying.
Deus Ex: Human Revolution (and to a lesser extent the first game, but I'm talking about DX:HR here) involves a relevant situation. Your pilot lands you and then gets unescapably trapped. She instructs you to make a run for it, and distracts the attackers who begin attacking her aircraft. At this poin there is no indication that there's anything else you can do but run away, and defeating the attackers and rescuing your pilot is almost prohibitively hard (even more incredibly so if you choose to do a pacifist run). But you can do that!
Finally, there's Spec Ops: The Line. A game about the brutality of war, and the brutality of FPS players who trivialise it with games :D. It wouldn't work if you were able to succeed at "nice" solutions.
There is a difference between a hard (but intended) path and a path that is impossible because the developers did not intend it io be possible. In your Bioshack example, yes you'd be weaker if you take the good route, but as long as clearing the game is still realistically possible while doing so, it doesn't seem as if you *must* follow this path.
Perhaps a better example of the difficulty of the "good" path could be in Ninja Gaiden for the NES. You are intended to use your sword or ninja techniques to kill enemies that get in your way, and the developers, of course, expect you to do this (and there doesn't seem to be any moral issue present here). However, if you restrict your killing to bosses and don't kill any normal enemies, it is still possible to complete the game. You will die a lot (especially with all the damage boosting you need to do; sometimes not getting hit isn't an option, or getting hit strategically is a better option than trying not to get hit) and probably need to use a lot of continues, but it is still possible to beat the game; it is not, however, how the developers intended you to play.
An interesting example to look at here is Undertale, which, as most people who've heard of the game, has 3 main routes.
* The neutral route, where you kill some enemies, but don't seek out every single one. This is the path of least resistance; it is the easiest route, and the path that players will likely end up on on their first playthrough.
* The pacifist route, where you don't kill anything. Since you can only level up by killing enemies, you will have to beat the game at level 1. It turns out, however, that the game is specifically designed to allow this; you never need to kill anyone, and because of a mechanic where having high current HP makes you take more damage, levels don't make as much of a difference as you might expect.
* The genocide route, where you kill everything. While this is an intended path through the game, the game does push back if you try to take this route. For one thing, when you kill most of the enemies in an area, the encounter rate drops, making it more tedious to find those last few enemies.. For another, while most of the bosses are trivial on this route, there are two bosses that are incredibly difficult. (Interestingly, the game provides an out befire the first of these bosses; in-between the save point and the boss, there is a trivial battle; if you choose to spare the enemy, you will leave the genocide route.) In this way, the devloper is providing this option without condoning it, in a sense.