Why the title reads as nanatsu no taizai? Not nanatsu no daizai? (CASE CLOSED)

Theo1899 said:
大 can be read both as tai and dai. No rendaku involved. In this case it's tai because... well it says so in the cover so it must be right.

Buexta said:
Both variations exist. Daizai is a serious crime, usually something that is punished by 10+ years of prison or death.
Taizai is a religious term. Translates into cardinal sin. That is why this series uses Taizai because the main characters are based on the seven cardinal sins of the Bible.

Black60Dragon said:
Welcome to the world of Kanji! You are just going to have to memorize pronunciations on a case-by-case basis. There's this thing called onyomi (Chinese pronunciation) and kunyomi (Japanese pronunciation). Depending on how the kanji is used the way it's said will change.

Typically, however, it follows a pattern like so:

1. If the kanji is by itself, use kunyomi
2. If the kanji is used in a compound word use onyomi

Of course some kanji even have multiple kunyomi or onyomi, but for the most part of that is the general rule. So yes like was already said 大 can be pronounced both ways. Hope this helps and good luck!