Fear is a very strong survival instinct and emotional energy but it is dangerous to feed on this.
When a nil or awakened human or even some newbie vampires feel fear they begin to retreat to a place deep inside their minds.
When we feed on their fear it causes their fear to increase to terror. The fear becomes too much for their fragile minds to handle, and it can cause any nil and even some newly awakened human minds to snap.
This drives them insane or makes them into, for lack of a better word, Rain Men, like in the movie Rain Man with Tom Cruise.
When we feed on peoples emotions they normally are not aware of it but when we feed on their fear they are very aware of it, and this causes a downward spiral into madness. They know we are the ones doing it which then makes them a threat.
They might attack out of terror just like a cornered and terrified animal will attack whatever has it cornered. Or they may let others know what we did and they may try to hunt us.
Just suffice it to say that fear is a bad emotion to feed on.
Fear is not the same as anxiety.