Atheists don't believe that there is no necessity for good behavior. Bill Maher is just as reviled by the murder and torture of innocents as you are. And may I just add that I find it increasingly more insulting when I hear that as an atheist I have no reason to believe in doing good. I don't look at the Holocaust and say "eh what's the big deal", I know that murder and genocide is wrong because it negatively impacts human flourishing. I can come to that conclusion on my own, as Mr. Maher has, without a divine entity telling me via scriptures written in the Iron Age.
When good happens in the world, who do you thank? It's though God's love, though you're personal relationship with Jesus/God right?
When something horrible happens, now we hear that "God works in mysterious ways". It's this double standard that Maher is bringing attention to.
The main problem with Maher's comment is that murder carries with it the implication of unjustifiable killing. So taking a certain story or certain statement by God,
without consideration for the context, including the rest of Scripture, not just the immediate context, one may not see that God is most certainly justified in what he commands. And this is a continual problem with atheists when they raise such supposed problems with the God of the Christian Bible--they don't understand how important context is, not that it is likely to satisfy them anyway when it is given.
Is God a "psychotic murderer"? Not at all, as he is just in his judgements. Maher just uses rhetoric, which will, unfortunately, convince many people who don't put much thought into what he says or don't actually look into what the Bible says for themselves. Perhaps Maher hasn't even actually looked into what the Bible says.
As for, say, something like the Holocaust, atheists have no ultimate grounds for calling it evil, as though it truly was evil, just as they have no grounds for calling something else good, as though it really were good. All such statements by atheists are purely subjective and boil down to personal preference--nothing is truly evil, it simply cannot be, it is just something that ultimately one finds distasteful. But of course, I do believe that (most?) atheists do, in fact, find that some things really are evil, such as the Holocaust, and this is because humans have the God-given ability to recognize the truly good and truly evil, based on an objective moral law. This points to a transcendent lawgiver.