First off let's start off with the fact that since Illumi was the first Zoldyck child he was most likely used as a lab rat for the Zoldyck parents to figure out new torture methods for their future children and how they tortured him so much he forgot what feelings are.
Second, can we talk about how people don't realize how much Illumi loves his brother Killua. Everybody says that Illumi abuses him but that's how he was taught to love because as a child when his parents would abuse him he probably thought that was them loving him and didn't realize they were using him as a lab rat.
Wow I just explained all of it in 109 words
