字 The braking sense is always shā: 刹车 (brake), 急刹车 (to brake hard), 手刹 (handbrake). The other reading, chà, is the Buddhist-temple and split-second sense, as in 刹那.
刂 on the right is the side form of 刀, the indexing radical. It groups 刹 with 切 to cut and 划 to slash; the blade reinforces the image of motion sliced clean to a halt.
杀 on the left supplies the shā sound and a violent edge. In the brake sense the violence is figurative: slamming a vehicle to a hard stop is treated as 'killing' its motion. It also serves as a strong intensifier in casual speech.
