Lesser Masticore does not have a -1/-1 counter on it.
Steps
Activate Altar of Dementia by sacrificing Lesser Masticore.
When Lesser Masticore dies, its persist ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it due to Curator Beastie.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to the sacrificed creature's power.