Tāmen qīfu nàge xiǎohái, wǒ kànbuxiàqù jiù chōng guòqù le.
They were bullying the kid - I couldn't stand it and rushed over.
Tips
grammar
Negative potential complement: V不下去 means "unable to continue V-ing". Compare 吃不下去 (can't keep eating), 读不下去 (can't get through reading). The positive form 看得下去 exists but is rarer - the pattern almost always appears negatively.
