Yeah. Since Killua is an assassin, he's also very cautious and pessimistic, as we have seen numerous times. That's why I thought, when in doubt, he would naturally go for a kill instead of mercy like a normal person, perhaps. It could be hanging around Gon, and the others, have mellowed Killua. It's not like he would have had a single friend before Gon, so he likely wouldn't have earlier thought somebody, let alone an enemy, could be cool and killing him might be a waste. An assassin would have finished the defeated opponent thinking it ensures the person would never appear before him again.