How awful!  
可怕噢!

That's horrible/dreadful!  
好可怕/吓人!

What a nightmare!  
真是恶梦一场! 

How terrifying!  
吓死人了!

That's scary!  
真令人害怕!

How frightening!  
恐怖噢!

How creepy!  
真令人毛骨悚然!

That's spooky!  
阴森森的!