What does this have to do with anything?

If you're on trial for murder, the prosecution would never call on you to testify. Why would they? Of course you're going to claim that you didn't do it. Your testimony would be useless.

Murderers are tried with physical evidence linking them to the crime, and witness testimony from other people. Murderers don't even need to be present to be convicted; this happens sometimes if they're considered a danger in the courtroom, or have fled.