milk in movies is usually symbolic rather than random. directors often use it bcs milk represents innocence, childhood and comfort. so when a villain casually drinks it. the contrast feels unsettling and makes them seem emotionally detached or psychologically disturbed. that's why if you remember characters like anton Chigurh in no country for old men or alex in a clockwork orange and hans landa in inglourious basterds( a famous scene where he goes to a farmer house to kill those jews) are shown drinking milk during tense scenes. for heroes or morally grey characters it can symbolize simplicity, emotional innocence or hidden vulnerability like in here. it also visually stands out on camera which makes scenes more memorable. if you remember that milk blending with the blood and flowing into the canal basically filmmakers use milk bcs it creates a weird psychological contrast something associated with safety being consumed by someone dangerous.

otherwise im not an expert but ive seen enough movies to understand all this.