"Let's join them!" Since the checkpoint has been breached, there is no need for Zhou Tian to stay behind. Even though there were no Beastman Warriors advancing during the previous battle, after experiencing that conflict, it's believed that the same Beastman Warriors will be much more composed when facing a similar situation next time. Thus, after sufficiently honing the skills of the Beastman Warriors, Zhou Tian finally launched a true attack on that checkpoint.
Once Zhou Tian got serious, those NPCs could no longer resist the invasion of the Beastman tribe.
After most of the NPCs in the checkpoint were removed, the remaining NPCs in this domain became dispensable. If Zhou Tian did not attack them, those NPCs could coexist peacefully. However, once Zhou Tian decided to take action against them, under the full strength attack of the Beastman tribe, even their weakest checkpoint, let alone the strongest one near the beginner area, probably wouldn't withstand Zhou Tian's invasion.