The first question to answer has to do with your premise. What exactly does the Bible have to say about… This is the root where most of the differing answers come from in determining what exactly the Bible means by what it says. The following is a quote from my notes on Hermeneutics:
It is impossible to come to the text of the Bible without preconceptions. Our language, our various cultures, our questions, our way of reasoning, what we have been taught, our history, all these and more will color what we read. Because of this, how we interpret the Bible will probably reflect just as much on who we are and who we are willing to become as on what the Bible actually means by what it says. We need to remember that we may start off approaching the Bible as a discerner of truth but, in the end, the Bible will be discern the truth about us. 
Does this mean that no one can know with confidence what the Bible means? Definitely not! My main premise is this: If we believe the Bible, the Holy Spirit can teach us, over time, how the Bible is to be interpreted and understood.
A lot of progressive Christians do not see the Bible in this light. They have been taught by many modern theologians and the influence of their own experiences to suspect the Bible, and what the Bible teaches, of subversion. This comes from a philosophical stream called Deconstructionism. “Deconstruction denotes the pursuing of the meaning of a text to the point of exposing the supposed contradictions and internal oppositions upon which it is founded—supposedly showing that those foundations are irreducibly complex, unstable, or impossible.” Because of this view the text of scripture is not deemed a reliable witness, but instead becomes a tool for the oppressor to dominate with.
The point of all this is that many people do not trust the scriptures. They feel that it was birthed in a patriarchal, ancient society, who had their own tribal god whom they exalted as God over all. Consequently, the goal of interpreting the scriptures becomes more of finding reasons why it doesn’t apply than to believe and do. And reasons can be found. Even Jesus had to ask his disciples if his teaching offended them to the point where they would leave him.
This cynicism in regard to the scripture does not provide an atmosphere wherein the Bible can be interpreted along the lines of its original intention. The question is, do you want to know what was originally intended? Or do you want to prevent the scriptures from influencing your life? Do you want the control?
Does this decision need to be made a priori? Not exactly. But the scripture should be approached with an openness towards the very real possibility that it is what it says and reads like it is. I call this a relational hermeneutic. It is like when meeting a new person. If we are prejudiced against them before we get to know them, we will never really know them. This does not mean that the scripture needs to be accepted without reasons. We need a working theory so to speak. I contend that that working theory needs to be one based on trust and faith instead of cynicism.
So this is how this works out in life. One of the questions you raised was what about those who teach that the God of the Old Testament and New Testament are not the same and that Jesus came to show us the difference. This, of course, is an effort to understand why the God of the OT could command apparent genocide while Jesus commands us to love our enemies. The different God solution was proposed very early on by a guy named Marcion. “Marcionism was an early Christian dualist belief system that originated in the teachings of Marcion of Sinope at Rome around the year 144.” And “Marcion believed Jesus was the savior sent by God, and Paul the Apostle was his chief apostle, but he rejected the Hebrew Bible and the God of Israel. Marcionists believed that the wrathful Hebrew God was a separate and lower entity than the all-forgiving God of the New Testament.” If we feel we can pick and choose our scriptures, we may, as did Marcion, chose to reject all scripture that appear to oppose our concept of love. This concept of love is one we have formulated outside of scripture’s influence. Mostly it comes from our own culture, which, since it is our own culture, we are probably unaware of its influence on our thinking.
If, instead of relying on our cultural worldviews, we allow the scripture to influence our thinking, we will come to different conclusions. If our concept of love is shaped by the entire corpus of scripture, we will see that although God is love and we are to love our enemies, judgment of sinful behavior is consistent with love. It is not loving for God to allow sinful behavior to go on and continue to hurt more and more people. This is why Jesus died on a cross and not in his sleep. This is why God has a day set aside for judgment. Judgment is one way in which God expresses his love.
You can see that in my view much of the progressive Christian theology has to do with applying a western cultural concept of love as a lens through which the Bible is read. This is close to what I believe is appropriate, since love is the great command and the very nature of God. But we don’t get to define love any old way we might want. Love is too important to leave up to us.
 “There is a judge for the one who rejects me and does not accept my words; that very word which I spoke will condemn him at the last day.” (John 12:48 NIV) “Speak and act as those who are going to be judged by the law that gives freedom.” (James 2:12 NIV)
 It has been suggested that “Neither the Bible as a whole or the New Testament in particular was meant to be a textbook on the science of hermeneutics.” A.J. Bandstra, “Interpretation in 1 Corinthians 10:1-11,” CTJ 6 (1971) 20. Cited in Biblical Exegesis in the Apostolic Period. R.N. Longenecker, (Eerdmans) pg.198. Of course the Bible was not meant to be a textbook on the science of hermeneutics. Yet those textbooks, when referring to the Bible, should elucidate the example of the Biblical writers hermeneutic. Paul told us that whatever we “heard from him keep as a pattern of sound teaching.” (2Tim 1:13 NIV) Who would you rather have as a teacher, Jesus, Paul, or some scholar? I would rather bet on Jesus and the Apostles being right. See New Horizons in Hermeneutics. Anthony C. Thiselton (Zondervan) pg.369
 John 6:60-61 (NIV) On hearing it, many of his disciples said, “This is a hard teaching. Who can accept it?”
Aware that his disciples were grumbling about this, Jesus said to them, “Does this offend you?
 “The terms "a priori" and "a posteriori" are used primarily to denote the foundations upon which a proposition is known. A given proposition is knowable a priori if it can be known independent of any experience other than the experience of learning the language in which the proposition is expressed, whereas a proposition that is knowable a posteriori is known on the basis of experience. For example, the proposition that all bachelors are unmarried is a priori, and the proposition that it is raining outside now is a posteriori.” http://www.iep.utm.edu/apriori/