I want to know if my understanding of the term やっちまった is correct. Could you please help me out?
The word used is やっちまった, this is the past form of やっちまう which means "to do something one regrets". It is a colloquialism built from やって, the -te form of やる (to do), and しまう (to finish).
However, colloquially, やる can mean "to harm; to injure; to kill" and しまう, after the -te form of a verb, means "to finish ...; to do ... completely", indicating completion (and sometimes reluctance, regret, etc.).
So やっちまった can mean "to completely kill" or something alike.