Clean up is an assumption. Peter Porco wore a painter overall while he attacked his parents with multiple axe blows - while they slept. Gloves and protective suit make cleanup easy. This was targeted, planned, pre-meditated. The suspect probably kept the knife not only because it is a tool, but because it is a trophy with memories.
The suspect could have stepped outside, removed gloves and overalls, put them into a plastic bag, sticking the knife into the dirt while walking away, and been long gone before the other 2 tenants woke up. The car ... it looks like the only mistake by the murderer, and clue. CCTV