This is the 15544th most frequent Russian word.
преступником
"Преступником" means "criminal."
In this sentence, 'преступником' is in instrumental case, describing the role or identity assigned to someone by the subject (the court).
Он был признан преступником судом.
He was recognized as a criminal by the court.
Here, 'преступником' is also in instrumental case, used with the verb 'считать' to describe what someone is considered as.
Мы не можем считать его преступником без доказательств.
We cannot consider him a criminal without evidence.
In this context, 'преступником' is in instrumental case, used after the verb 'называть' to denote the role assigned to the subject.