Boy, these pups sure are a handful huh? No Niju's not being evvvvil here, that's his son Niko...And let's just say Niju has good reason for his harshness.
Niko pulled a Simba and almost got him and some of his siblings (Adak, Yakone and Spica) killed by a bear. They went on an adventure and the pack set out to find them after they were gone for way too long. Niju, Nuk and Yak were looking for them in a group when they hear the pups screaming for help. After rescuing them, it became apparent that Niko had been the one who came up with the idea and pushed everyone into it.
Thus he gets in the most trouble- rightly so. It turns out, Niju is probably yelling at Niko about how horrible his grandfather (Niju's dad) was-and how Niko must avoid ever becoming him. Niju's father is more evident in Yakone it turns out...but more on that later. At this age, Niko is the bully of the pups, and Niju is very aware of that.
At least Niko obviously feels remorse.