Killua is shown throughout the series as a strong character, very cocky and powerful and by far the strongest of the 4 Leads before Nen is introduced by miles but the best thing about him is how flawed he is. He has trouble really accepting people sometimes and holds very little remorse when it comes to murder. Gon is the only thing that keeps him on the edge and he has constant fears that he isn't good enough, worrying that Gon has more potential than him, and he has shown himself when in a rage to not be able to hold back.
He is a bad ass to the extreme. You don't mess with this this kid. He is a master of some sweet assassination techniques which he shows off in a couple episodes. Him and Gon make a good pair, I wouldn't mess with them.