。
She reported him to the police as a murderer.
Literal
She [topic-は] him [object-を] murderer [as-として] police [to-に] reported.
~として ('as, in the capacity of') qualifies the role someone is being assigned — here, casting him as a murderer. 訴える is multipurpose ('to sue, accuse, appeal to, report'), and context decides which sense applies; with 警察に + ~を + ~として, it lands on 'report/accuse to the police as.' 人殺し (literally 'person-killer') is the colloquial, blunter word for murderer — its formal/legal counterpart would be 殺人犯 ('homicide perpetrator').