Ahh yes, no wax in a time candles would exist? Makes sense.
Also even if by some strange reason they hadn't figured out wax... Oil and animal fat also work as a polish for skin and hair and many other things, you aren't going to tell me oil and animal fat doesn't exist as well right?
For the killing kids thing, I don't think it was necessary after they were already incapacitated, either way they would have died from the other monsters while unconscious or ran back when woke to start a manhunt. Either way he would have had to leave. I can only assume the author wanted to show that the mc lacked empathy and fundamental emotions that make one more "human".