You want to argue with God, go right ahead.    But, ok I'll play God's advocate for a bit even though I am woefully unqualified. I'm still more qualified than you are...obviously.   First, they aren't demons but flawed humans--sinners like we all are.   Second, what's wrong with it is the example of Paul himself--there is a possibility of repentance and salvation that you would cut short if you killed the sinner for whom Christ died.   So, let's go with a less Biblical approach. What if...the law of karma was a real thing? What if God's call for us to forgive others has more practical merit than you can understand? If you forgive, instead of keeping the wheel of karma going, it ends. If you kill someone, they get to kill you in the next life.   It comes down to faith. Do you believe you're all that smart? Or is there a (very damned good) chance that the author of the Word knows more than you do? To be fair, I've thrown my share of bricks at the Temple myself. But in the end, I'm only human.