When I receive a subpoena to appear in court as a witness to a felony case, can I not show up?

If you were properly served, then you must show up or you risk the court issuing a "body attachment" whereby the sheriff comes out to take you into custody and delivers you to the courthouse, and then the judge will order you detained until you have completed your testimony. Are you sure you want to risk that, just for personal reasons? If the reason is that you fear retaliation from the defendant, then you need to contact the prosecutor's office and police to discuss your fears.

If you were served with the subpoena, you need to appear or risk being taken into custody.

No. If you are properly served you must appear. If you don't you can be held in contempt with a warrant issued for your arrest.