User blog comment:AustinDR/PE Proposal: Slappy the Dummy/@comment-34055410-20200105172113/@comment-1705775-20200105173119

don't forget that Slappy is by far one of Stine's more constantly wicked and detailed monsters, who is given a very clear personality and purpose.. whenever Slappy appears everyone knows things are going to go very bad, even other Goosebumps antagonists regard Slappy with some degree of respect.. those who don't are rare cases of being Bigger Bads (like the Horrors) and even then Slappy only ever allies with others for as long as it suits him, the moment a larger threat is dealt with Slappy will betray you.

Slappy is also, as Austin noted, on a more depraved level than most of the other Goosebumps monsters because he outright wants a child-bride and everyone else is to be either his slave or killed - being a recurring characters aid him greatly too as he can amass a much larger array of feats and he's also constantly one of the more openly violent antagonists.