It should surely be OK if they use deception to get you to reveal information that only the perpetrator could know. Like "We know you buried the body in that forest [lie], so tell us which tree it's under." And you tell them which tree, and they find the body there. Unless there's some weird circumstances, that proves you knew where the body was buried and should be valid evidence contributing to the conclusion that you murdered them or were at least an accomplice.

Seems like just applying honest common sense and intelligent reasoning solves this, and I thought that was the judge's job. Maybe judges are the problem for not seeing through the bullshit.