OK - first (grin) you are correct about the original languages. Imagine me - unfamiliar with the original languages but very well educated and an amateur Bible scholar - trying to use the original language text to unearth a translation that is better (more accurate) than a translation that was created by an entire TEAM of accredited, experienced scholars paid to do that very thing.
Would you trust me if I told you " 'witch' is not the correct translation - don't believe the scholars who have spent years earning a doctorate in ancient Hebrew, additional years gaining experience in translating Biblical Hebrew, and have succeeded so far in their efforts that people are willing to pay them to be part of a team translating Hebrew. Believe me instead."?
So the original language thing - though it **can** be very useful in gaining a more *precise* translation, is usually no help in getting a more accurate translation unless you have earned a doctorate in the appropriate language yourself.
So - how to know if a translation can be trusted, short of spending the next decade or so earning doctorates in Hebrew, Aramaic and Greek?
Very nearly all modern Bibles are translated from the most reliable, critically-edited original language source texts available. Nearly all modern Bibles are translated from these peerless source texts by one or more teams of appropriately-accredited scholars. So, very nearly all modern Bibles are *extremely* trustworthy (particularly in comparison to older Bibles) - but they all have shortcomings, and here is a list of a few of them:
* bias - some include very obvious sectarian bias; some are very "ecumenical", which means that very many Christian sects that are very different in teachings had representatives vote on which precise wording was used in the Bible - a sort of democratic Bible. Some - the best - are less ecumenical but concerned primarily with conveying the original meaning rather than with making the Bible agree with what they believe. It's not always easy to tell them apart - especially since the reader (you or I) is *also* biased.
* translation oversight: some Bibles are translated by an individual (the worst case); some are translated by committee, which means that no one man decides the wording of any one verse and that it must be agreed upon by several scholars; some are translated by multiple committees, each responsible for a particular portion of the Bible that is the field of their specialty.
* inclusive language: this is for people who are concerned with very specific issues regarding Biblical doctrine. In the original languages of the Bible, as in English until about 30 years ago, standard practice was to use the masculine pronoun when the author meant *either* a man or woman. Example: "A driver should bring his car to a complete stop." - the old method in English, and the method common to Biblical languages. Clearly, "a driver" might be a man OR a woman, but the grammatical convention in such cases is to use the masculine.
As you can imagine, sometimes it's tricky to determine if the author meant "men only" or "either men or women". In the Bible, when translators use words that mean "man or woman" in places where the original language uses only the masculine is called "inclusive language". Some Bibles are very conservative, only using inclusive language when that meaning is absolutely certain. Too conservative an approach actually leads to problems in accurately conveying the meaning. Other Bibles are very liberal in using inclusive language - some intentionally use it in every place that it might possibly be correct, even when it is not likely that it is correct. The best Bibles are pretty careful about using it ONLY when it is *very likely* that it should be used. The reason this is best is that it allows the reader to determine if inclusive language is meant where the question is uncertain - that is, it leaves the interpretation up to the reader.
Now - here is a chart that displays all of this info for some of the more popular Bibles and many of the best Bibles available in English today.
http://www.bible-reviews.com/charts_accuracy.html
What you will find is that all of the most highly-regarded Bibles - from a scholarship standpoint - use "witch", "sorceress", etc. instead of "poisoner" in Exo 22:18. You have, it seems, been misled on that score. If you look up the word in a Hebrew lexicon (the word is transliterated "kashaph", and it is identified by Strong's number 3784), you will find that "poisoner" is not a definition commonly accepted by Hebrew scholars. Here's one such:
http://www.bibletools.org/index.cfm/fuseaction/Lexicon.show/ID/H3784/kashaph.htm
On the other hand, you are largely correct about "thou shalt not kill". Although it certainly does NOT mean "you shall not murder in cold blood", what it does mean is "you shall not murder" - or, to word it another way, "you shall not kill another human illegally". Notice that "thou shalt not kill" is *VERY* close to the more precise meaning, but that the more precise translation is - obviously - the better translation.
So: which version? I'm afraid that you have not given us enough information. Still, this web site will help you to narrow down the versions to just a few
http://www.BibleSelector.com/
when you've done that, you can contact me (here or from that web site) and I can give you a full run-down of the pros and cons of the few versions you are considering. If that's too much work, the best (most accurate) English Bible at the current time in my opinion is the New Jerusalem Bible. There are other excellent translations, though - so I really recommend that you step through the above web site and narrow down your choices before coming to a final decision.
Jim