Christians that justify their decisions or actions by using a single sentence, phrase, or word from the Bible are often guilty of ignoring the essence of the message out of a desire for misdirection or intentional ignorance. A closely related approach is when the reader interprets the importance of a Bible passage grossly out of proportion to the overall message as a justification.
By way of example, is killing wrong? What about killing an assailant to protect another? Or, killing in war? What is the higher directive based on Jesus' example and the Bible's teaching, protecting others or avoiding murder?
It is harder, or at least requires more investigation, research, and thought, to understand the big messages. Messages from the Bible that are not from a single phrase but developed over many examples and extensive passages. It is easy, and often a misinterpretation, to pick out a single phrase and claim to follow "Christian practice" by only acting consistent with the one phrase rather than the entire body of teaching provided in the Bible. This notion is similar to the difference between an expert's views and a novice's (see: How is an expert's knowledge different from novice's).
13 Thou shalt not kill.
Tho shalt not kill. But there's killing by Christians documented throughout the Bible. So is it okay to kill in some cases? If so, what are the cases? Is it a static known list or flexible and developed by the participant in real-time? Turns out, this is not a simple question resolved by one sentence. As with many such topics the deeper truth is only realized by deep thought informed by reading and study.
The New Testament also has passages that specifically speak to this issue. Given that "Christians" are attempting to live a "Christ like" life (after all it's in the name), then the best guidance comes from reading about Jesus' life in the New Testament of the Bible.
In Luke, we read about the Pharisees being accused of following the letter of the law (like following the specific phase or sentence in the Bible) but not the broader teaching that is the purpose of the law.
Luke 11 - King James Version
42 But woe unto you, Pharisees! for ye tithe mint and rue and all manner of herbs, and pass over judgment and the love of God: these ought ye to have done, and not to leave the other undone.
Luke 11 - New International Version
42 Woe to you Pharisees, because you give God a tenth of your mint, rue and all other kinds of garden herbs, but you neglect justice and the love of God. You should have practiced the latter without leaving the former undone.
Then in Mark, Jesus provides another example both in words and deed.
3:1: And he entered again into the synagogue; and there was a man there which had a withered hand.
3:2: And they watched him, whether he would heal him on the sabbath day; that they might accuse him.
3:3: And he saith unto the man which had the withered hand, Stand forth.
3:4: And he saith unto them, Is it lawful to do good on the sabbath days, or to do evil? to save life, or to kill? But they held their peace.
3:5: And when he had looked round about on them with anger, being grieved for the hardness of their hearts, he saith unto the man, Stretch forth thine hand. And he stretched it out: and his hand was restored whole as the other.
3:6: And the Pharisees went forth, and straightway took counsel with the Herodians against him, how they might destroy him.
Jesus performs a healing on the sabbath where, by law, no such "work" is to be done on that day. Is the rule or law the important deciding factor or the general Christian principle that we should help others, and particularly the least among us? By his action, Jesus was demonstrating the answer.