There is a difference here in David's time, men were sometimes ruthless enemies. We can have some principles in our country today that we may not be very able to apply to Christians who live in Muslim or Communism dominated lands. Most of the time they are as ruthless against Christians as this psalm describes.
In the 16th chapter of John, Jesus describes that there will come a time when killing a Christian will be looked at as doing God a service. We are living in that day particularly with the persecution that is affecting Christians around the world. Perhaps we may even come to experience such a thing in our so called peaceful country, although the actual murder may never happen. Now we jump up to verse 12 of this psalm which says that the LORD will maintain the course of the afflicted. We have seen this at times in our country today and we also see it on occasion in lands where persecution is prevalent. We must stay with God come what may.