yeah, the law is too stupid to notice broken pinkys. id take the heart only on full moons or something like that, and do some other weird shit every other week, and the signature woud be a phrase or symbol cut into the same place on each vic. like the all seeing eye. that would freak people out.
That's a tough scenario. With modern forensics alone, you wouldn't stand much of a chance with getting away with it no one seriously implicating you as a suspect. If you managed to somehow pull that off, you'd still come off very suspicious.
There are a few approaches you could do, but Occum's razor slides both ways. Cops want a clear, easy, air-tight case. Murders are rarely elaborate, even the premeditated variety.
Method 1: "Self-Defense"
The Body: Leave it alone, for the most part. At the very least, don't move it, and don't try to clean anything.
Motive: Jealousy - You tell the police that you and the friend were seeing each other, and that one time she came over and saw another of your lady friends parked outside your apartment and figured you were cheating. Since you didn't consider your hook ups anything more than that and she did, she flipped out when confronting you. She was enraged that you wouldn't treat her like your girlfriend or even consider going steady. Don't try to act like a perfect saint/victim. Explain that there was definitely a fight and arguing before "she lost it". This one is tricky, because they'll ask about times you two got together, and any correspondance you had for setting up plans, like emails, texts, or calls. You'll have to explain that the first time it happened was a get together, and that after coming back for more, you left her your key to drop by. Push the fact that there weren't calls or texts because you were afraid that your sister might catch on, and since you were fuck buddies (in your eyes) you didn't want to even deal with that kind of drama. Take a copy of the key, clean it, then rub it on her hands to pick up dna and partial prints, and slip it on her key ring. The more sleezy you seem, the better. The cops will have a reason to hate you, as will your sister, but better a sleezeball typical guy than a murderer. People have attempted murder for less in crimes of passion.
Crime scene modification: Wreck the place quickly and quitely, at least where any of her blood or your blood might be. Broken glasses, knocked over furniture, et cetera. If you make too much noise after the kill, little tommy might fuck you over when they question him.
The "Self-Defense": A little self mutilation can go a long way to help sell the point, but you have to know what you're doing. You can't just take a piece of glass and cut yourself a few times, you have to be smart about it. You're going to have to bruise yourself and slide bits of glass into the fake impact area. Another good thing would be making it clear that this wasn't just a brawl of rage, but she was trying to kill you. Depending on your Fat/Muscle ratio, a few self-inflicted gut and chest wounds that go in to the layer of fat but not much else can definitely help your case. Grab a belt to bite into to help you get through the pain and not make too much noise when you're doing it. Wounds are not as simple as an entry. You're going to have to have an angular exit slide of the weapon, even if it is minor. Just remember, the less things that crime techs have to analyze, the less shit you have to explain away and hope they don't call bullshit on. A good head gash from an impact in a struggle can also do wonders to ham it up. You can look like a bloody mess with a relatively harmless head wound on your forehead and hairline. You'll look like a gory mess when the cops arrive and you can simply explain it as her opening attack taking you by surprise and throwing you into, say, the table or wall or something. If you're ballsy enough to give yourself a concussion/contussion to make it seem more legit, more power to you, though I'm not sure it would be necessary. It would serve as a decent excuse for your state of mind though, and any confusion you might conveniently have while be questioned. Plus, it would mean that you'd have priority medical attention before being questioned, and give you time to brainstorm out your story for the sequence of events.
Sequence of events: Keep this short and sweet. No epic battle, nothing fancy, just a quick struggle that scared you and ended tragically.
KISS: Keep it simple stupid - the less fishy your story seems and the more willing you are to cooperate, the less priority the case will have, and the less the techs will comb over the evidence. You might get time for manslaughter or some other charge, but you probably won't get murder. If the DA or the detectives imply that they want you for anything more than self-defense, that's the time to lawyer up. If you do it right away, it's going to be suspicious, and if you don't do it for whatever charges they decide to pursue, you could get really fucked over with your sentancing.