The very first thing he did when he took the stand on cross from the DA was to admit that he had lied about a material fact of the case, allowing the jury to be instructed that the rest of his testimony was untrustworthy. He did a pretty spectacularly bad job on the stand.

I'm surprised he went down for murder 1, but there was a pretty damning array of things lined up against him. Aspergers or not, he admittedly ripped up and flooded his car while trying to actively hide it from the police in a storage locker; he claimed a 6-inch(!)-wide blood stain on a bag (not a sleeping bag, but its pouch) in that car was from having sex; and finally, he built his case not on the premise that he didn't kill Nina, but that she was still alive --- a very simple assertion for the DA to knock down.

Once you take the stand, you have no control over how long you will be on the stand. The fifth amendment insures you don't have to testify if it will hurt your case...but if you agree to be questioned, both sides get to ask pretty much all the questions they want within the bounds of the law and the patience of the judge.

But, yes, it's clear that him taking the stand was the worst possible thing he could do and he apparently fought loudly and repeatedly with his lawyer over this decision.