Wildalaska, I believe you CAN invoke the 5th amendment protections agains self-incrimination even after taking the stand. It just LOOKS really bad to do that in front of a jury.
You are technically correct to a limited extent, but Wildalaska is more correct. A defendant can take the stand, be sworn, and refuse to answer questions, but no one would do that. As you say, It is really bad to do that in front of a jury, and the jury will correctly infer that your answers really would incriminate you. It amounts to an in-court, voluntary confession.
Once the defendant testifies, he waives the 5A protection against self-incrimination as to anything he has testified to or otherwise "opened the door" to with his testimony. And his credibility may be impeached, as with any witness, by showing he has made prior inconsistent statements (and if relevant, prior convictions and, sometimes, prior bad acts).
So taking the stand is risky, particularly so with this defendant, who I think will have trouble connecting with the jury on an emotional level.
OTOH, the defendant may have to testify as to his confusion and explain away as best as possible why his prior statements were inconsistent and hope the jury comes back with a lesser included offense, such as manslaughter.
Some of you may remember Dan White, the ex-supervisor of the SF board of supervisors (city council), who sneaked into city hall with a gun and murdered the mayor, George Moscone, and fellow supervisor (and gay activist), Harvey Milk. He was convicted of manslaughter (triggering a riot among the gay population in SF). If not,here's a link:
http://en.wikipedia.org/wiki/Moscone-Milk_assassinations