"Who defines what is evil and what is not?"
To the nontheist, the consensus sensibilities of society (which are fluid over time). To the theist, the source documents which advise the dogmatic beliefs of their faith (which are much less fluid over time.)
"If you study the history of Christianity throughout the ages, you can quickly see a massive amount of evil, and not much good."
If I watch the evening newscast, I see a massive amount of evil, and not much good. If I read ANY history, I read a lot of evil, and not much good.
Why? Because evil fascinates us. Because it fascinates us, it makes its way into the evening newscast and into history books. Good, which is boring, does not.
Hence, your statement *assumes* without reason that the recorded history of Christianity (and my religion, Islam, for that matter) is entirely accurate and balanced to both the good and the evil done in the names of both those traditions. This assumption is fundamentally flawed. History is (a) written by the winners, and (b) usually records fascinating occurrences whilst leaving out that which is not fascinating.
"It all started to go south at the Council of Nicaea in 325 AD where the Christian Gnostics and their writings were banned for political reasons, and out of thousands of writings, this group of men decided what would constitute the cannon, which became the Bible with all of its translational and intentional flaws."
True. But the Christians believe that the Rudder (which includes the proceedings of all the concilar actions) were guided to correctness by God. You (and the authors of Britannica) do not. Hence, their view is biased in favor of the accuracy and purity of the proceedings, whilst yours and Britannica is biased, leading to the description you outline above.
There is no way to determine whose view is accurate and whose is not. Neither science nor logic provides sufficient tools to make that determination.
"I do understand that many of the Christian values have percolated into the public sphere, but there are some Christian values that are actually evil. "
By today's consensus, quite true.
"but they should inspect them individually to keep the good, enhance the mediocre, and throw out the bad."
Without objection. However, the question always remains if the consensus of society is true or false. As you're quite aware, there was a day in the not so distance past where the consensus of the USA was that black slavery was morally acceptable. That consensus changed, as mores are wont to do.
"Look at the white Christian people who are openly racist and want a return to the lily white fake Christian America that never existed except in their little rural communities. Yep that is a good Christian value to inspect."
I'm not so sure this is a good example. In this example, one can easily make the case that Christian racists hold their positions in opposition to their own source documents. Indeed, this was the case in the pre-Civil War period, where Northern evangelical communities pointed out clearly from Scripture that slavery as the South practiced it was in violation of Scripture, forcing the Southern evangelical communities to prooftext references to slavery whilst ignoring the humanitarian teachings.
Regarding your proposed thought experiment regarding precepts, I agree that theists should indeed apply such tools, but instead of considering "consequences" (after all, neither Judaism, Christianity, nor Islam accepts end-justifies-means rationalization) they should instead determine what in-context passages in their source documents justify that belief or behavior. I think it entirely appropriate for nontheists to demand that theists show proof that a particular belief is indeed an essential part of their faith. THAT SAID, it is NOT appropriate (or even wise) for the nontheist to demand that the theist re-evaluate a well documented belief in an attempt to force it to match today's consensus sensibilities. (IOW, when a nonMuslim tries to tell me what my own religion teaches, it rarely ends well.)