The problem with those people is that they think they know the alternative.
There is no such thing as a what-if box. This is an imaginary box that would allow him to see what would have happened if he would have killed Jesus. This man just presumes that the world would be a better place if Jesus would never have been born. I can assure you the contrary. If Jesus would never been born then we would be all slaves now, or humanity would have already eliminated itself.
This is the same effect as you look at the stock market and think: If I would have bought here and sold there, then now I would have been a millionaire. And you look at the stock trends and you feel sure, but the problem is that you look at the past trends where you are tricked into feeling to imagine yourself to know the future from a past standpoint. But when you start buying stocks, then you understand it is just a higher form of gambling with slightly different odds where you will probably loose everything at some point. So it is with this man wanting to eliminate Jesus. He sees mankind's trend and thinks it could be better and then blames the influencer. But he doesn't know the alternative.