Bewitching Xie Yan Er and his followers to slaughter each other was indeed Xiao Ming's intended outcome.
But of course, this was not because he was particularly sinister or a demon from hell, but rather because he was very clear in his mind that to extract the truth from the lips of an experienced spy, one must first make the opponent's defenses crumble.
The so-called conquest of the heart doesn't necessarily have to target the person directly, sometimes targeting those close to them is more effective.
Unless Xie Yan Er was an emotionless psychopath, the outcome of personally killing a companion, with whom he had spent days and nights together, would certainly cause cracks in his emotional defenses.
Both outcomes meant death, but betrayal and personally taking action are of two completely different natures.
As for Si killing Xie Yan Er, that was simply not possible.