For the non-believer, sin is non-existent. The idea of "sin" is really about the distribution of good to bad - which, as we can see through history, is purely subjective over times and cultures. If a commandment from the OT states "thou shall not kill", it can only be understood by people on a subjective spectrum, not as some objective absolute that is well defined. In modern western culture, self-defense is justifiable even if you end up killing another human. But the commandment is not clear at all in the matter - otherwise, an all powerful god would certainly make that very clear: "thou shall not kill, unless it is unavoidable in self-defense". If one determines that "kill" is actually "murder", then that implies some level of pre-meditation, which is illegal in most societies today - regardless of any religious terminology. In fact, if most bibles simply stated "thou shall not murder, nor be complicit in the murder of another person or persons", I would completely agree with that very sensible "commandment".
However, can you imagine a scenario where Jesus would use self-defense to kill someone? Neither can I. In fact, carrying a concealed weapon in order to "defend" ones self is probably the last thing Jesus would do. So, we have massive subjectivity and disagreement over the entire set of "laws, rules, commandments" from within the very ranks of YHWH/Jesus, and all of the supposed authorities of scripture over time. Humans are making it all up as we go.
One of the very first and most horrific abuses of YHWH in the Bible is when he explicitly murders everyone with a violent flood (according to the myth). To claim that the law-giver is free from guilt in this case is avoiding the very heart of the commandment. The commandment does not say "thou shall not kill, only I, YHWH, can do so". In what scenario is it ever OK for the lawmakers to break the laws with impunity? Perhaps in the case where the law-maker is also the creator? However, this is precisely why many people cannot accept any claim that the creator that behaves this way is a loving creator. There is simply no way to connect a loving creator, to one who is willing to kill all humans because it is not pleased with how its human creations are behaving.
As far as the tribes that YHWH commanded to have wiped out, there is no difference in the outcome. People who believe that YHWH is the creator, acted on his ruling and behalf, swinging their swords down upon the helpless bodies of infants, children, women, the elderly, and everyone else in that tribe that displeased YHWH. The humans carried out the killings, which were nothing short of pre-meditated murder - a direct violation of the commandment. Again, the commandment does not say "thous shall not kill, unless I command you to".
Modern Christians cannot defend this at all. There is simply no way to separate and distinguish the "will of some invisible creator that no one understands" to human ideas of morality and laws. The history of scripture being used and abused is nothing more than a river of blood. No amount of apologetics and special pleading will change this.
Any Christian today, when confronted, would be forced to admit that if YHWH commanded them to slaughter a baby, they would have no choice but to do so, yet those same Christians would also tell us that YHWH would never do such a thing, which is obviously wrong.