Answer:Sirius was attempting to break into the Gryffindor common room to find and kill Peter Pettigrew (Wormtail), who had been disguised as Ron's pet rat, Scabbers for the past twelve years.
Following on from this answer, when the Fat Lady would not give him access to the tower because he didn't have the password, Sirius (not Lupin) became angry and slashed the painting in an attempt to get her to open and allow him access. However, she still refused him access and then left the portrait, essentially sealing off the common room.
Answer: Sirius was attempting to break into the Gryffindor common room to find and kill Peter Pettigrew (Wormtail), who had been disguised as Ron's pet rat, Scabbers for the past twelve years.
raywest ★
Following on from this answer, when the Fat Lady would not give him access to the tower because he didn't have the password, Sirius (not Lupin) became angry and slashed the painting in an attempt to get her to open and allow him access. However, she still refused him access and then left the portrait, essentially sealing off the common room.
Ssiscool ★