Killing is criminal and wrong.
And we as outsiders do not know what actually transpired between the parties, leading on to the killing in the end.
But having said that, for those who outright called the killer uncle siaolang, OCD or intolerant, have a look at the available information.
Neighbours had only good things to say of the uncle. He was described as friendly and helpful, with no known anger management issues nor complaint about neighbours. If the said uncle was indeed a siaolang, then during the decades he has been living there, he would have complained about his upstairs old neighbour already. Something might have triggered his complaints and then killing only after the victim's family moved in 2 years ago. What caused the changes?
And for those who said with sweeping statements that kids are like that one, and those who complained must not have kids themselves: bear in mind, there are different kinds of kids and parents. Some parents would teach their kids the right thing and what not to do to cause nuisance to others (like the EDMWer in an earlier post), while others simply disregard others and forget 己所不欲 勿施於人. There are really some idiotic neighbours who create noises that torment their downstairs neighbours for years; those who experienced it will know.