Jeongmin still has a high degree of self-control. Siyun has demonstrated he can't properly control himself anymore (it's not just that he used a concealed weapon but the fact he was very pleased with his actions). Jeongmin is stuck between a rock and a hard place because she is essentially a leash on Siyun (which is not a good relationship or partnership to have long term). There may even come a point Siyun attacks Jeongmin (as strange as this sounds, this is a common pattern in domestic violence cases, especially those that result in murder)