Could have been another fight club
Spoiler to fight club below....
Other than a few small details like the other cop car chasing keanu into the gas station and keanu going to marisa tomeis office it seemed like all the interactions were just the two of them so it could have all been in spader's head and he was the real killer. That would fit with spader being kind of nutty and also why keanu was doing those freaky dance moves because he was another personality of spader. The body in the water could have been a patsy as it was burned too badly to see the face.