This phrase doesn't typically refer to actual murder, but to someone avoiding consequences for bad behavior. Even though he broke the rules, he seems to get away with murder.
Pro Tip 2/3
Not Literal
Don't be confused; it's usually not about a real crime. It's an exaggeration to express frustration over unfairness. She's always late but never in trouble. She gets away with murder.
Pro Tip 3/3
Negative Connotation
The phrase has a negative tone, showing disapproval of the lack of consequences. Since he's the coach's son, he often gets away with murder on the team.