OME things are terrible. Technically, this means they in some way bring terror. Yet, over the years the English word "terrible" has been polluted and diluted until we even use it casually to describe a flat cola as it sits warm and fizzless in the hot sun on a picnic table. That is hardly the same meaning that David intended in Psalm 66:3 & 5 where he described God as a "terrible" deity. David meant that God is worthy to be feared and to be revered. He is awesome and dreadful. He should be honored and respected. And indeed, for those who are not under the blood and in His favor, meeting Him will be a terrifying experience.
So, God is great and wonderful... His abilities and accomplishments are amazing. And, if we are against Him, instead of with him, His positively amazing and wonderfully appealing qualities seem to morph before our eyes into frighteningly troublesome characteristics. Hence, theologically, fear has two faces... one with a worshipful smile, and another twisted into a horrified scream. Whether we are obedient or rebellious determines which brand of fear fits our features. But God is the same in either case.
Of course, even when we are walking with Him, His ways can sometimes be terrifying. Psalm 66:10-12 tells us that sometimes he tries us like silver (with heat) and lays affliction on us. Sometimes His ways are terrible (terrifying) in our eyes, even when He is doing us good and exercising us with love and approval (Hebrews 11:36-38).