the fact you don't know is why you must see the doctor

As an ex-cutter myself... I find it scary you cannot figure out why it is bad for you. As griffin said... that is why you should see someone. It doesn't take a lot of brain power to realize causing physical damage to your body is bad for you. A child could figure that out. I'm not saying you need to stop.... but you should maybe be a little less ignorant.

You're putting a knife into your skin. It should be self explainatory as to why it's bad for you Seeing a doctor would be the best thing to do. But if you're so set on not seeing one, I would look for other ways online to relax and relieve stress and anger Punching/screaming into a pillow comes to mind

To start off with, any time you break your skin you open up the possibility of infection. There are staph bacteria normally living on your skin. As long as they are on your skin instead of in your skin, they do no harm. But once the skin is breached the bacteria can get into your body and may even cause life threatening systemic infections. That is what normal staph can do, but if you get one of the superbugs, which are everywhere now, it can kill you in days. MRSA (methecillin resistant staphalococcus aureas) is particularly deadly. Next point: skin that is repeatedly injured in the same place can develop cancer. Third point: if you slip and cut too deeply, you can nick a tendon and cause it to rupture. That happened to a friend of mine, accidentally, just a few months ago. She is finally out of the cast and still doing physical therapy to regain full use of her hand. At least that was an accident, she bumped a sharp knife in a dishpan of soapy water. It would be worse if you'd been dumb enough to do it intentionally. What you need to do is learn to deflect anger and to look at it critically. The very fact that you blew up at the other poster tells me you don't have that skill. You didn't even take the time to notice that the poster was trying to help you. All you noticed was that you didn't get the answer you wanted and you immediately went to anger because it was the only tool in your toolbox. You need some other tools.

Why it's bad for you- Scars you will have to look at the rest of your life, chance of infection, cutting to deep and hitting an artery(bleeding to death) and last but not least, you are still not dealing with anger so you are starting a cycle that really can't end. Face the anger and let it out.