In the darkness, Lew Kuan Yee had sworn revenge against Michael, the detective who had sent him behind bars.

behind 在这里是介词，表示"在...后面"或"在...之后"。 bars 是 bar 的复数形式，指"栏杆、栅栏"，在监狱语境中特指"牢房的铁栅栏"。 behind bars 是固定短语，字面意思是"在铁栅栏后面"，引申为"被关在监狱里"的意思。