What is that one crime scene/detective movie?

it has been killing me so i thought i would ask the internet world, but all i could remember from this movie is that these 2 guys wanted to just kill someone so they got this girl walking back from her car at a grocery store. and they did there homework and there was almost no trace leading to them, and they hide out in the treehouse thing and were gonna kill themselves but each person thought that the other person ws turning on them...what movie is this?