I've beaten the game, and I can report - choosing the evilest option available at all times while doing the main quest will leave you with slightly good karma right up until the final decision (pre "Lost Chapters" content), which, by itself is sufficient to push you most of the way towards maximum bad karma. Once you're in the "Lost Chapters" proper, there are plenty of opportunities to make evil decisions, and your karma never has a chance to recover.
It's actually a very odd design for a karma system. Weighting it so heavily towards the end means that your evil character is going to spend most of the game mostly good. Plus, actually choosing the evil path means your character acts like a raging moron. The big decision that pushes you deep into bad karma is whether you want, after killing Jack of Blades, the man who burned Oakvale at the beginning of the game and make you an orphan, to claim the mighty sword of Aeons that he'd plotted so hard to acquire. Doing so means you have to kill your sister, who you'd previously assumed was dead and only recently rescued from the clutches of Twinblade's bandits.
But what, really, is the reward for this? You get a blade that is admittedly more powerful than the one you already own, just after beating your worst enemy, and only about an hour before you get an even more powerful sword by entering one of the expansion pack's new demon doors. Similarly, there is a chain of quests in the "Lost Chapters" where you have to collect souls to open a certain door, and the evil path involves killing all your allies because it's slightly more convenient than finding available souls. At one point in the evil path, you actually have to kill someone who is doing vital translation work for you, with no indication that there is a replacement available.
It's kind of ridiculous. Which I'm fine with. Realistically, an "evil" character should be nuanced, doing things for many of the same motives as a "good" character, but being just slightly too . . . flexible with the means. Then you'd have a person who leaves a trail of human wreckage behind them wherever they go, but who will, when confronted with their acts, close ranks and attempt to defend their actions.
Killing Briar Rose because otherwise you'd have to walk ten feet does not fit that rubric. I can't even begin to imagine how you'd come up with a rationalization for this one.
You could call it shallow, but I think it's just as likely that it's a manifestation of Fable's cheeky sense of humor. The difference between evil and good mostly boils down to whether the player presses a certain button, and thus is, in essence, cosmetic, so the "evil" playthrough is just the "good" one with an over-the-top nasty coat of paint (supporting evidence for this theory - your alignment can be swayed by up to 25% of the total bar just by wearing the right outfit). Evil is defined by senseless audacity.
Which I guess could be fun, though I think I much prefer the good playthrough, because then at least the story makes sense.
From here, I'll probably start a new file as a good character, and perhaps focus on being a magic user. This time out, I was primarily a front-line fighter, who leaned heavily on the Physical Shield spell. Fable veterans will realize that I'm basically confessing to being a real-life Jack of Blades here, but I'll explain for the uninitiated. One of Fable's keystone mechanics is the "combat multiplier." The more hits you land without getting hit yourself, the more your combat multiplier increases. Your combat multiplier multiplies all the experience points you received by its rating. The Physical Shield spell surrounds you with an aura of blue light that causes attacks to damage your mana points instead of your health points. As long as it is active, your combat modifier will not be reduced by hits. And it only ends when you run out of mana. And mana potions are just as fast to drink, and just as cheap, as health potions.
The practical upshot - I was able to max out all my relevant stats about 12 hours into the game. The only thing I had to buy at the end was the fourth rank of the Berserker spell, which I could not previously access thanks to being "neutral" for most of the game. It's a naked exploit, and I probably should be ashamed, but it made the game super easy (I made it through the whole thing without using a single resurrection vial).
I guess I figured if I was going to play an evil character, I might as well act like an evil player. Mwa, ha, ha!
I won't have that advantage as a mage (you need too many mp to cast your spells). It ought to be interesting. I hope I haven't allowed my near invincibility to make me lazy.