It doesn't work like that

Here is a free tip to potential murderers. People seldom, if ever, stab themselves 31 times. As a defence against murder, it’s pretty lame. In fact, the only people who stab anyone that many times are seriously whacko. Specifically, the only cases I’m aware of which feature that sort of overkill involve an offender who is seriously mentally ill, a serial killer, or possibly both.

Actually, without giving away too many secrets, there are features which are characteristics of self-inflicted stabbing injuries that make them relatively easy to distinguish from stab wounds inflicted by someone else. So stabbing someone and then claiming they did it themselves is a pretty good way to get yourself caught.