He's not comically cruel, he will go along with a goody-two-shoes party and even help save Tieflings. Tagging along with a group of powerful individuals serves his own purpose, which is to deal with the mortal peril that is the tadpole in his brain.
His entire concern is purely his own wellbeing and power.
That's exactly what I mean, an evil character like Astarion will work well in groups or with a "good" playthru, since they will still work towards a common goal with the group or towards the "good" ending and accept being the hero. The issue is more the kind of evil that makes its own plans, so that the campaign events become nothing but the backdrop to their own schemes. This can be a murder hobo, but also the kind of slow plotter who wants to bring down the old order and grab power. The tadpole scenario allowed for a broader range of alignments to fight towards a common goal, but with your "normal" campaign, an actively evil character would increasingly become the main villain. Not causing the goblin attacks, those just came in handy, but using the goblin attacks and an own false flag operation against their border keep to discredit the mountain guards and rise as a warlord. Or maybe they want to subjugate the area for their cult or get rid of human presence in the mountains. Of course, there are synergies - the dragon who drove those goblins out of their homelands is no longer a boss fight, but a potential ally - but you'll have to create different opposition and hopefully have a resourceful player who can surprise you. It's a different campaign. That's all fine if you know it beforehand, but having to write a game without knowing what exactly an individual villain may do, is really tricky.
