It is an unfortunate fact that throughout human history, all the way to the modern day, people who have claimed to be God's followers have committed a variety of atrocities. This has lead to a number of unbelievers believing Christianity must be an evil religion. But the real question isn't whether or not Christians do bad things, but whether or not these bad things are consistent with the Christian faith.
In the above image, I have included a freshly fired gun. This is because by far the most common example is murder. Whether war, persecution, or even just an individual murderer, the fact is that people who have committed some of the most gruesome murders in history have been identified, whether by their own words or not, and whether they genuinely were Christians or not, as Christians. So let's take that as an example. In fact, let's take the most common example: Adolf Hitler.
The question as to whether or not Hitler was a Christian has been addressed by this ministry, and other ministries far greater than my own, a number of times. The drum has been beaten so many times that its skin will snap if so much as a hair touches it. No, he was not. As a matter of fact, he called Judeo Christianity a pestilence that would soon meet its end. Nevertheless, misotheists continue to claim he was a Christian. So, we'll be extremely generous. We'll accept, for sake of argument, that Hitler claimed, both in public and in private, that he was a Christian. We can even take a picture of him and photoshop a little cross around his neck and a Bible in his hand. Now for the real question: Would this matter? Not even remotely.
As violent, perverse, or profane as nominal Christians may have been for the past 2,000 years, the fact remains that none of them define Christianity. Quite the opposite, Christianity defines its followers. In the entire history of the human race, the actions of only one man, Jesus Christ, have any bearing on Christianity. This is because not only is it His religion, but He is also the only one the religion acknowledges as sinless, the perfect role model, and worthy of our obedience. In other words, unless you can point to anything Jesus did that is evil, or anything that justifies the evil actions of other Christians (I'll save you some time: It's not in there), there are no actions you can point to to blame Christianity for evil, even if that evil is committed by a genuine Christian.
Obviously, if an authority figure says "yes", and their follower does not obey, the authority figure is not to blame, but the follower is to blame for disobeying the authority figure. Similarly, if the authority figure says "no", and the follower does it anyway, the authority figure is not to blame because the follower disobeyed. In Christianity, the Bible is the sole and sufficient authority. If the Bible condemns a thing, but a Christian does it anyway, neither the Bible, nor the God who inspired it, are to blame. Likewise, if the Bible commands a thing, but a Christian fails to do it, neither the Bible, nor the God who inspired it, are to blame. Only the Christian who disobeys is to blame. The argument from evil Christians, therefore, is completely invalid.