Possible film names

True Love

This name is good because it relates to love in the sense that Hermann is in love with Diane, however is his love true? Or is he just a psychopath? It defies the conventions of the film as the title is contradictory to the contents of the film. 

Bloody Love

I like this name as it relates to love in the same way as the above title but it suggests that there is going to be trials that involve death or blood in some way. It doesn't tell us why there will be blood, if it's to do with reaching love in the stereotypical way of the guy having to save the girl or in some other strange way. 
I decided to go for this name.

Stalker

This title states that a main character in the film is stalking one or more of the other characters. It doesn't give a lot of information about the film which is good as people will have to look into it to see what it is about and then be intrigued to watch it.

Valentine's Day

Valentine's day is good because it makes you think that the film is going to be happy and romantic, however it is quite the opposite as the film is about a stalker who basically kidnaps Diane and tortures her.