The verse says:
" When thy Lord inspired the angels, (saying): I am with you. So make those who believe stand firm. I will throw fear into the hearts of those who disbelieve. Then smite the necks and smite of them each finger. " (8:12)
The Mekkans tortured, killed confiscated the money of lots of the Muslims in Mecca so they emigrated to Medinah. The battle of Badr was the first real battle between Muslims and the pagans of Mecca. This verse (8:12) was speaking about the war of Badr. At this battle the pagan army consisted of 950 fighters and 314 so the forces were not even close and of course don't forget the weapons and horses which was very large in numbers for the Meccans and much less with the Muslims.
It has been narrated on the authority of 'Umar b. al-Khattab who said: When it was the day on which the Battle of Badr was fought, the Messenger of Allah (may peace be upon him) cast a glance at the infidels, and they were one thousand while his own Companions were three hundred and nineteen. The Holy Prophet (may peace be upon him) turned (his face) towards the Qibla Then he stretched his hands and began his supplication to his Lord:" O Allah, accomplish for me what Thou hast promised to me. O Allah, bring about what Thou hast promised to me. O Allah, if this small band of Muslims is destroyed. Thou will not be worshipped on this earth."
He continued his supplication to his Lord, stretching his hands, facing the Qibla, until his mantle slipped down from his shoulders. So Abu Bakr came to him, picked up his mantle and put it on his shoulders. Then he embraced him from behind and said: "Prophet of Allah, this prayer of yours to your Lord will suffice you, and He will fulfill for you what He has promised you." So Allah, the Glorious and Exalted, revealed (the Qur'anic verse):" When ye appealed to your Lord for help, He responded to your call (saying): I will help you with one thousand angels coming in succession." (8:9). To encourage Muslims to regain their right, Allah was encouraging them by saying that he will help them in this unequal battle by terror that He will implant in the heart of the unbelievers as well as the angels who will fight with the Muslims.
So the verse was for that particular occasion of a WAR in a BATTLE FIELD against an enemy who is much stronger than you and who treated you unjustly before and was so full of grudge against you, not for anyone to use out of context to justify their barbaric deeds against civilians who did not hurt you in any way.