Give some stream of consciousness and then the answer. Jimmy and his friends get Dave drunk at a local bar. When Dave leaves the bar, the men follow him out. Jimmy tells Dave that he shot "Just Ray" Harris at that same location for ratting him out and sending him to jail. Jimmy informs Dave that his wife thinks he murdered Katie and tells Dave he will let him live if he confesses. Dave repeatedly tells Jimmy that he did kill someone but it was not Katie: he beat a child molester to death after finding him having sex with a child prostitute in a car. Jimmy does not believe Dave's claim and threatens him with a knife. When Dave finally admits to killing Katie thinking he can escape with his life, Jimmy kills him and disposes of his body in the adjacent Mystic River.

Based on this passage, who killed katie in the movie mystic river?
To answer the question, consider the following: When Dave finally admits to killing Katie thinking he can escape with his life, Jimmy kills him and disposes of his body in the adjacent Mystic River.
Therefore, the answer is Dave.