Try "He is a policeman, but quickly becoming a robber". Many policemen make up the police department, but there is no such thing as 'a police'.

With regards to the next sentence, if the target sentence can be restated as "He is one of our colleagues but sometimes he may be an enemy", the notion 'sometimes' and 'quickly becoming' are not equal. Sometimes implies two state are possible. 'Quickly becoming' implies a one directional change from state ‘A’ to state ‘B’.