Good point. God exercised His sovereign justice on the Canaanite kingdoms. He did not give such power to mankind to determine such justice. He is Deity we are not.
Exactly. The law he gave us after the Flood is DO NOT SHED BLOOD. DO NOT KILL.
So, therefore, when you have a religious movement killing people, you know - automatically and without further thought required - that it has careened off into evil.
And if it can be THAT wrong on something THAT fundamental, you cannot place much reliance in anything else it says. You have to examine it all, and judge it all, and decide, yourself, on your own sovereign authority as a creation of God, what you will accept and what you reject.
And if somebody then screams you're a heretic, you shrug your shoulders. If he picks up a sword, or a torch, or a thumbscrew, then you know for sure he's an agent of the Devil - and you can defend yourself against him. And if his INSTITUTION defends HIM in HIS violence, then you know that the institution is infected with the Devil and cannot be relied upon for the unvarnished truth either.
In the end, you are the judge of all churches, all books, every religion - every order given to you - because in the end you are the only authority that moves any of your limbs.
Put differently, the arm that swung the pistol into place and blew off the head of the Jewish prisoner in the Holocaust could have just as easily swiveled in place and blown off the head of the Gestapo Major ordering the execution. In either case that arm, and the brain that powered it, is dead today - and alive somewhere else. The arm that blew off the head of his commanding officer is the one that is not writhing in flames today. The ones who obeyed orders are.
At least if God is just and what he is said to have said in the much-vaunted Scriptures is actually true.
If it isn't, well, then it's every man for himself - and the men who can best cooperate with each other dominate the rest - which explains why Christianity dominated in the disorderly West, but why Islam crushed out Christianity in the East and North Africa. In the West, the Christian hands held the swords against wild barbarians, so it meant civilization. But in the East, Christian hands held the whip of slavery, and the Muslims freed the slaves who converted.
What is evil where depends not on what men believe, but on what they do. Acts are what matter - words are wind.