Network intrusion detection systems (NIDS) are essential for web security by identifying and dropping malicious traffic. Existing in-network NIDS leverage the Tbps-level packet processing capability of programmable switches to achieve high-speed flow classification. They translate complex trained machine learning models to decision trees (DTs), where DTs are deployed on programmable switches via single-DT or multiple-DT deployment. However, they face a fundamental trade-off: single-DT deployment suffers from low classification accuracy due to over-pruning of trees, while multiple-DT deployment suffers from high overhead due to deploying multiple tree replicas. In this paper, we propose Proteus, an in-network malicious traffic detection system that achieves both high classification accuracy and low overhead. Its key idea is to split the original DT into critical and normal sub-trees, where these sub-trees have different impacts on overall accuracy. More precisely, Proteus first splits a DT into one critical and several normal sub-trees for adapting to the accuracy requirement and switch resource budgets. Second, it minimizes coordination overhead between sub-trees while ensuring full flow coverage via mixed-integer linear programming. Third, it dynamically reallocates or migrates sub-trees to adapt to changing resources by monitoring both classification accuracy and switch resource changes. Testbed experiments with 12.8 Tbps programmable switches show that Proteus improves classification accuracy, reduces switch resource consumption, and reduces classification latency.