Shar wants me to do evil acts.
No, Shar wants you to do particular things that she is interested in, whether those things are good or evil is subjective.
And of course alignment should open up and close off certain conversation threads.
You can't accommodate for all the logic any individual could follow, why would you reduce roleplay value based on your arbitrary interpritation of someones motives?
1) Save the tieflings. [ Good ] ( They've done nothing wrong and don't deserve to die. )
2) Save the tieflings [ Evil ] ( I think tieflings are evil Devil spawns and I want to watch the world burn. )
3) Kill the tieflings [ Good ] ( I think tieflings are evil Devil spawns and I will rid the world of them. )
4) Kill the tieflings [ Evil ] ( I just like killing. )
I can come up with ANY alignment motivations for ANY action, every dialogue choice would have to have like 9+ variants of the SAME option in different flavors, OR we can just roleplay in our head what makes sense.