How can Christians say with a straight face that their God is not capable of doing anything bad?

You guys have read the Bible, right? We are talking about Yahweh, aren't we? I just want to make sure we're talking about the same god, here. Because Yahweh sent flood that killed millions! And he destroyed cities where lots of people lived, even children were there, just because he didn't agree with their lifestyle! I don't know, maybe you guys have a different definition of what bad is, than I do, because that all seems pretty bad to me!