Simply tell them that because of your lingering pain, you don't feel that you can be objective. That's honest enough.
If they tend to pry further, tell them you aren't comfortable talking about this personal matter, which should underscore your first reasoning. If that's not enough, see a doctor, and have your physician state that depression can alter decision in such a state. I, myself, would not feel comfortable passing a life altering judgement on anyone when I felt I was not at my best intellectually.
Just be honest. Tell them how you feel.
They will more than likely ask you what emotional turmoil you have been through, and that is when you say you consider it a private matter, (that's also when the doctor note comes in handy as there are doctor/patient confidentiality laws) and that the cirumstances have left you questioning your capability to be logical - and perhaps even cause a lack of the incliniation to care - or the opposite, depending on the trial - which, in your opinion (and mine) would prove to be unfair to the person on trial, as it could possibly lend to prejudice regarding your ultimate judgement.
Good luck with the legalese, John. And my sympathy goes with you regarding your emotional pain.
I understand - and I too believe that the accused should have the right of objectivity.
and hugs, too, btw.
May you heal quickly.