判 is used in: 判断 (to judge/determine), 判决 (verdict), 裁判 (referee/judge), 批判 (to criticize), 谈判 (negotiation). It conveys the idea of making a judgment or distinction.
Knife radical on the right - the side-form of 刀, Kangxi #18. Supplies the cutting/judging action: a verdict 'cuts' the case open. Same radical in 别 (separate), 刚 (just-then, originally hard-cut), 列 (line up by cutting), 刻 (carve).
Left side 半 (half) supplies the sound: bàn → pàn with regular b-/p- alternation. 半 also carries direct semantic weight here - to judge or arbitrate is to 'cut a case in two,' splitting right from wrong. Phonetic and meaning reinforce each other.
