Lolth is the deity of law turned evil. Strict, unforgiving, unbending, and fully prideful.
Her purpose in a campaign is to create an implacable enemy. She is the supreme and sole ruler of the Underdark and has total sway over Drow Society.
Her power is such that direct confrontation is impossible. In my campaigns, all Drow are lawful evil, because they are tied into Lolth’s strict power structure and are ultimately controlled by her.
When Drow break away they are usually true neutral or chaotic evil. Occasionally chaotic good like Bel.