When you read those quotes ... and its not even everything ... You will find out that some people litteraly did.
"Rewards" does not mean gold and swords, but it could mean a more solid lead on fixing the tadpole problem or giving you stronger allies down the line. The evil route does not do that at the moment, the goblins always betray you and Minthra often does too. Halsin is objectively your best bet, because Minthara, Gut and the goblins all want to kill you. The player knows this even without meta knowledge. A smart evil player would therefore always side with the Tieflings; they just have the most to offer and stand the least in your way.
Honestly Kagha as a third option would already solve a lot, if Zevlor didn't ape jump you when you tell him it's time to leave. When you betray him at the gate, the player says "sorry, but Minthara and her cult are the best option I have". When was this established? They are hostile to you all the time, show they know nothing about the tadpole and Halsin clearly does. He even studied one up close. What in all of that incentivises the player to murder a group of innocent refugees for some hostile tadpole denialists that betray you, other than "ooh I'm evil!"?