>A guy with a hatred for fanatical zealots
Goes on a campaign of mass murder. Also prevents history that causes billions of people to never have been born.
Will he also kill Marx? Marx's followers managed to kill ten times as many people as Mohammed and Jesus put together. And they each had thousands of years of head start.
The thing is, mass murder happens. Fanaticism happens. These things happen, and they'll always find a pretext to do these things under. It might be religion, politics, assumptions of cultural superiority.
You live in a very rare moment of peace, prosperity, liberty, and justice. Vanishingly rare in history, rare even in this era. So you assume that it's the natural state of things, and if it weren't for a few Really Bad Guys that everything would be great. The truth is, the natural state of humanity is violence, poverty, oppression, and exploitation. Only by constant vigilance and sacrifice do we hold that state at bay.
One of the pillars that keeps people from descending into total savagery is religion. It's sometimes a pretext for doing harm, but it also does considerable good.
It's ironic that people are so comfortable and insulated that inevitably a generation arises that overthrows their world, trading away the minor injustices that remain for the greater injustices of a failed state or dictatorship.
So if your character somehow did manage to get his wish (probably impossible no matter how many people he kills), he'd only succeed in cementing in place a uniform, ignorant, grey despotism.
Your character also prevents his own existence the first time he changes history, of course.