Also, burning someone alive is probably no worse than using spells like acid arrow, dolorous decay, fire storm, summon insects, cloud kill, death cloud, insect plague, creeping doom, poison, meteor swarm, or incindiary cloud to kill your enemies.
But the point is that you use such spells on
enemies during combat. If you go around using them on captive, helpless individuals who've never caused you harm and you do not know for a fact has ever caused anyone else harm, you most certainly are not a Good person.
Just knowing that the person deserved it would be enough for a chaotic character not to step in.
Believing that someone deserves a horrific death merely because of their race and the deity they worship, rather than because of actions you've observed them commit or it has been proven that they've committed, is not a Good belief. Good drow such as Drizzt Do'Urden and the followers of Eilistrae are well known to the people of the Realms, as is the fact that just as some Good gods have non-Good followers, some Evil gods have non-Evil worshippers.
Priests of Shar are also really, really bad news.
Being a priest of an Evil god does not automatically make the priest guilty of horrific actions. What if they've only recently joined the church, and have yet to commit any sort of criminal act? Being Chaotic doesn't automatically mean you don't believe in the concept of redemption.
Also, some evil gods, such as Talos, Umberlee and Xvim, even accept non-Evil priests, and since it's doubtful there exists in the Realms a list of which gods accept what type of priests, for all the player knows Viconia may not neccessarily be evil. Also, you seem to be forgetting that the people about to burn Viconia are priests of Beshaba (and declare themselves as such), meaning they themselves are Evil.
Basically, unlawfully killing someone - or allowing that someone to be unlawfully killed - based purely upon your own personal assumptions rather than strong evidence or hard fact is not a Good act. Letting people who are Evil themselves murder someone isn't a Good act either.
As a final note, Chaotic and Unlawful aren't the same thing. Being Chaotic means you lack self discipline and are likely to act upon whims, it doesn't neccessarily mean you have a total disregard for the law. Both the elves and those drow whom live in Lloth controlled cities, are 9/10 Chaotic aligned, yet both clearly have a very strict set of rules and regulations which they adhere too... though in the latter's case this is admittedly because if they don't, they'll end up dead.