There is something else to consider when looking at Jesus' teaching concerning loving your enemy. If Jesus is teaching a governmental pacifism then of course that contradicts the Old Testament. However, if that were true, it would be the only one of Jesus' teaching here where he is contradicting an Old Testament passage rather than a traditional misinterpretation/misapplication of Scripture or expanded custom.
In other words, Jesus is contradicting the tradition of the elders/Pharisees in their view of righteousness. He isn't contradicting the Old Testament, as many a modern interpreter has superficially concluded. They're all traditions or speculations concerning what Scripture does say, but Christ tells them that their traditions are wrong. He never does contradict the actual Old Testament.
Hence, if the pacifists were correct, this would be the only place in His entire Sermon, out of all of the examples given, where He outright contradicts the Old Testament itself, and this even after He explicitly says He did not come to do away with a single jot or tittle of it.