Voyeur
/vwɑːˈjɜːr/
Definition 1 of 3
Pro Tip 1/3Privacy Invasion
Using 'voyeur' indicates wrongful invasion of someone's private moments. 
Installing secret cameras is a voyeur's tactic.
Pro Tip 2/3Negative Connotation
'Voyeur' carries a negative judgment, suggesting disapproval of the act. 
Her roommate's voyeur habits made her feel unsafe.
Pro Tip 3/3Not Just Visual
Being a voyeur can also imply enjoying others' intimate auditory experiences. 
The thin walls turned him into an unwilling voyeur.