User blog comment:Silva92/Killua's Nen/@comment-14902757-20130909043126

Wow, I've been asking about this myself for some time now, can't believe I'll find the same question here. So as for what I've arrived for a conclusion, I think Silva left the obligation and style of Killua's training all to Illumi. And Silva probably accounted Illumi for Killua's life that's why he planted that thing on Killua. (Sorry again, I don't read the manga, so you might easily refute this with manga facts). As for why Illumi didn't teach Kil nen? He was about to but Kil run away. He's also too busy with his own stuff (like getting his own hunter license and with assignments given to him by Zeno and Silva) that's why he wasn't able to teach him right away. He knows how dangerous Nen could be and he didn't want Killua to learn things without his supervision because he also wanted Kil to be under his check and under the Zoldyck family's control.