There really isn’t a whole lot that you can do or say to influence someone who is coming at the subject of God in such a biased manner. Probably the best thing that you can do is be patient and ask some pointed questions to maybe get them out of the mode of attack. Then perhaps you can engage in a meaningful conversation. With regard to the killing in the Old Testament, warfare has always been a part of human history, still is. So it is a bit strange that someone would suddenly look at depictions of warfare in the Bible as somehow paradoxical. It only shows exactly what we have said all along, we live in a fallen and sinful world. The next thing that I would point out is that the narrative portions of the Bible such as Joshua, Judges, etc., are just that, they are narratives. They are not meant to moralize. So just because they describe a battle, or some other disturbing event, that does not necessarily mean it is upholding that event as a virtue. Many times the opposite is true. Lastly, with regard to the wars of the Israelites under Moses and Joshua (I am assuming that this is what Bill Maher is referencing), I would look at Deuteronomy 9 with regard to why the Israelites were told to dispossess the Canaanites. First, God had given it to them by promise to Abraham. However, in Deuteronomy 9, God warns Israel it is not because of the righteousness of Israel that has thrust out the Canaanites, but because of their wickedness of the Canaanites that God was giving Israel the promised land. These are a people who sacrificed their children to their gods, instituted sex slavery and prostitution as part of their fertility rites, etc. Something that should give our modern society some pause. In addition, God later warns Israel they will suffer the same fate if they are unfaithful to his covenant. A warning which came true.