SSo how would a defendant ever testify on his own behalf?
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I said nothing about testimony at trial, only self-serving statements.
A defense attorney can certainly put a defendant on the stand and ask him anything he wants to....but that then opens up the opportunity for the prosecution to tear the statements apart, which is why so many defendants are not called to testify.
None of Zimmerman's testimony about the incident is at risk of being attacked as inadmissible, on account of it being self-serving. Not the statements he made to police, not the phone call to police dispatch, not any deposition testimony he might give, and not any statements he makes at trial if the prosecution goes to that step. The fact that much of his testimony is in his favor ("self serving" in the normal use of English) does not make it inadmissible as "self serving hearsay testimony", a legal principle.
A hypothetical example of self-serving hearsay testimony that would be inadmissible would be Zimmerman attributing knowledge to others (which is a form of hearsay), such as saying "my mother knows that Martin hit me and knocked me to the ground." The law would admit the mother giving evidence like that, if the mother had actual knowledge; but in this case the mother's testimony would be ruled hearsay because she was not present, and thereby lacked first-hand knowledge.
All testimony is subject to being rejected by a fact finder as being not credible; but that consideration happens after the evidence is admitted.