Plastic wastes were catalytically transformed into different structured carbons as effective catalysts for peroxymonosulfate activation to degrade organic pollutants in water and the catalytic conversion mechanism was comprehensively investigated. A salt template-based carbonization approach was successfully developed to catalytically converting high-density polyethylene (HDPE) into diverse carbon materials, such as core-shell carbon composites, nanosheets, and their hybrids. The morphology and proportions of structure defective carbon were found to be controlled by a NiCl2 to HDPE ratio. Carbon nanosheets performed excellent catalytic efficiency in peroxymonosulfate activation toward phenol oxidation, due to a high content of reactive defects via a nonradical electron-transfer mechanism. More importantly, deliberate experiment design and kinetic analyses were employed to illustrate the artifact of radical scavengers (e.g., ethanol) in mechanistic investigation for nonradical/radical reaction. This work provides an upcycle approach for waste plastics into carbocatalysts and new insight to the conversion process and advanced water purification.