I’m on trial for murder. I don’t like my chances, so I’ll just stay home instead. They can’t make me show up, so I’m home free!
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.
From a quick search, it appears that a trial in the US cannot begin without the defendant present, although it may proceed if they leave afterwards depending on circumstances.