Last scene of "The Departed" symbolic... what you guys think?

Dignam kills Sullivan in his appartment in the end wearing jump suit and in the end we see a view of the balcony and a rat runs across do you guys get the same feeling as me that it was meant to symbolise that Dignam was the "real rat" because Sullivan did kill Costello not only to hide his identity but also I felt he wanted to get out of the irish mob?