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Since I found that many of ways to expand my Max source point pool behave a little evil, I didn't choose to expand it.
(I also think that the source skill is really so so, that I'm strong enough to drop it....)
I wounder if there would be any difference for the Ending?
I don't know about ruining the game, but there are other source skills that are pretty nice. Also, nothing in the game is so cut and dry as good or evil. I think the trick is finding the balance of play that suits you. You can probably go full evil, and leave nothing in your wake, or remain as lawful as possible, but you'll still probably end up as lawful neutral at best, but I'm still not sure who makes the laws since you start out in chains. The only way I could see that working is if you never removed your source collar.
Spoilers ahead:


from what I remember, your choices are...

1) Siphon the life of woodland critters, killing them all (and permanently losing a talent point if you have pet pal, like goddamn I hated that)
2) Eat a voidwoken heart
3) Kiss a succubus
4) Deepen your connection to the trees
5) Steal the life-force of a legion of spirits, erasing them forever


Now, of those, 1) and 5) are the easily identifiable as evil, but 4) is completely innocent. In addition, 2) and 3) have no negative effects on others, but might have RP limitations.

Given that you only need two, seems pretty easy to remain "good".
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