They are the ones that need us, not the other way around. It's because we got the key to all the doors, right in our palm. Without us, they can't close that Rift.
Mutual need still leaves the possibility that you could be compelled. If you make yourself a real pain to work with, maybe they tie you up and get you addicted to something. Use that leverage to control you. Maybe they don't even need to control you, Dorian seemed to be casting a spell that you were a component in - he just seemed to need your presence. Maybe they break your arms and legs and cart you around in a case to the sites they need rifts closed. Unless you've got The Litany of Adralla on hand, maybe they have a blood mage take control of you - I don't recall entirely how that's meant to work and I'm unsure if closing the rifts requires some active mental component on your part, but - just throwing that out there. They might be pious but they're not necessarily above the occassional dirty deed if you push them.
One of the problems with a character that's omnimalevolent is that the responses to it that might take place in real life are generally unworkable. If your choices keep hurting people, then the obvious move on their part is going to be either to not deal with you or, if they have to or there's some significant reason to, to heavily reduce your ability to make a free choice. If there's no serious threat that they'll succeed in reducing your agency, then you're in a world of cardboard cutouts. However, that runs contrary to good gameplay if all, or most of, your mechanics and content are geared towards enabling a particular class of choice.
In either case; a world of cutouts or a world where the answer is essentially 'you lose'; it seems likely to cripple the experience.
That's more managable in P&P RPGs, where there's some distance that can be had in roleplaying a character with constrained actions. But without creating an entirely different set of game mechanics, I don't see how it would work well in a CRPG.
That's not to say that the occassional evil action can't fit in, that all the party members would object to you deciding not to deal with L at an earlier point perhaps, or that you might find a party that just doesn't care if you're evil to others. However, that's not something that I feel can be adequately judged based on highly abreviated versions of a couple of levels of one dialogue tree.