Give the guy some slack guys, if you live peacefully for your whole life then suddenly you had to kill someone for survival it's a very drastic change. I too would be shocked to live where pizza and cup ramen doesn't exists. But yeah Ren really got to change his attitude if he wants to keep living.
Wouldn't it be funny if the inventions of the prehistoric times is actually brought by modern humans that got time travelled.
Also that would make it a paradox