The problem with the claim that Christ is good, but Christianity or the Christian God is not is this:

1) Jesus and God mutually approve of everything the other does. For that matter, they actually don't act that different per say. Old Testament God isn't really that vengeful (for example, in the book of Judges, he repeatedly forgives the nation for its own transgressions and seems to have an attitude of "I'll always take you back"), nor was Christ/New Testament God really all that merciful (in the book of revelation, he kills innumerable amounts of people. In fact, he's possibly more violent during this period of time than old testament God ever was)

2) Christians seek to be like Jesus. If you have complaints about the ways Christians generally behave, but have no such complaints with Christ, then your complaint is more that they aren't Christian enough.