Many have claimed that the God of the Old Testament was mean, capricious, vindictive, petty, unjust and much, much more. This usually stems from passages having to do with the flood, the destruction of Sodom and Gomorrah, the cleansing of the Promised Land of other nations and tribes, etc.
People will refer to these parts of the Bible and claim that God is a horrible, mean, violent deity. So much so that some people will claim that, even if they were convinced that this God exists, they would not follow such a being. They simply don't like Him. They don't believe such a God exists, but hate Him anyway.