Por que Aldo Raine tinha essa cicatriz?

Não é explicado nem no filme nem no exterior. Não há entrevistas disponíveis do elenco ou da equipe, há apenas suposições e teorias do usuário. Um pode ser encontrado em IMDb :

It is never explained in the movie, as it was Tarantino's intention to leave it unknown. Several theories have come up about it, one such being that it resulted from his punishment for being a bootlegger in the Southern United States. There is a rumor that Tarantino said in an interview that Raine was lynched by the KKK for defending African Americans in the South. It is also been suggested that someone slit or tried to slit his throat but didn't go as deeply as intended or that Aldo fought them off quickly enough, although the scar does not look like that caused by a knife, and the screenplay refers to it as a 'rope burn'.

Ele sobreviveu a um linchamento

Não se trata apenas de especulação, ou seja, do script :

CHAPTER TWO

"INGLORIOUS BASTERDS"

FADE UP

EXT - SOMEWHERE IN ENGLAND - DAY
A bunch of SOLDIERS are lined up at attention.
LIEUTENANT ALDO.RAINE, a hillbilly from the mountains of
Tennessee, walks down the line. He recruits the men, the
Germans will later call; "The Basterds". Lt.Aldo has one
defining physical characteristic, a ROPE BURN around his
neck. As if once upon a time,. he survived a LYNCHING.
The scar will never once be mentioned.

LT.ALDO
My name is Lt.Aldo Raine, and I'm
puttin together a special team.

Isso pode ser porque ele faz parte do índio americano

Às 1:20 , ele diz

And I am the direct descendent of the mountain man, Jim Bridger.
That means I got a little Injun^‡ in me.

Uma implicação disso pode ser que uma tentativa de linchá-lo foi feita com motivações racistas, mas isso é especulação .

‡ Injun: indiano, ou nativo americano.

Em IMDb é dito que

The reason for the scar on Aldo Raine's neck is not mentioned in the film. The script hints that Raine survived a lynching, a common punishment in the 1920s and 1930s.

E além disso , que é uma referência ao personagem juiz Roy Feijão do filme A Vida e Tempos do Juiz Roy Bean .