Hop Pop is not a killer, certainly not if you know how much he cares about his family. It could be something to do with the plot of the season 1 episode 'Snow Day', when all te frogs freeze, one of them always dissaepers and gets eaten. Wouldn't suprise me if that happend. Or maybe in a futher episode we will learn that their parents, of at least their mother maybe got killed by the toads or something to do with the king. I believe Sprig's role will get much bigger and important in the series along the girls. Like he is a link to something and Hop-Pop tries to protect him from that, explaining why he buried the box