That tells us about the act of spanking itself, not the motivations of the spanker (which is what's relevant for determining if one is a "monster").
The negative consequences on mental development of the child caused by spanking are great, which results in more spanking incidents in the future. Combine that with the fact that there are alternate approaches to dealing with these situations which have the same immediate effect with less developmental damage, and it becomes pretty clear that spanking is an unjustifiable act.
Now that we've established that spanking is unjustifiable in terms of its costs and benefits, and we know that it is an act of aggression (as, after all, it causes harm), we can state that the act of spanking is unjustifiable aggression.
Unjustifiable aggression is a violation of the non-aggression moral principle. Therefore, it is an immoral act. Immoral acts are monstrous acts, and those who perform monstrous acts are monstrous.