叛

Right 反 - historically the 又 hand radical with an extra stroke marking a hand flipped over, hence 'to turn against, to revolt.' The indexing radical here is 又 (Kangxi #29) inside 反, marking hand-turning action. Combined with 半 'split,' the picture is unmistakable: half-splitting turned against you = treason.

Left 半 supplies the sound - bàn shifting to pàn with consonant change. It also carries a sharp semantic flavor: 'to split or divide in two,' which is exactly what a traitor does - breaks off from the group, splits the loyalty. Same phonetic family: 伴 companion, 拌 mix, 绊 trip up, 胖 fat.