I honestly don't think it'd be different. People would still wage wars and kill in the name of other things. I mean there have been cases where sports rivalries have gone way too far (Giants and Dodgers) just to name a few.
Religion itself is not the problem, the problem are those who take it so deep to the point where they feel the need to shove it down everyone else's throats who don't agree and take it so personal when they tell them to Fuck Off.
I consider myself a devout Roman Catholic, but I've got know problem with Judaism, Buddishm, or Islam. I'm actually quite fascinated by Buddhism. Each one serves it's purpose.
But every single one of them follows the one coom on rule: the Golden Rule - And that is respect thy neighbor, or Respect the thoughts, feelings, and beliefs of others around you as much as you would like yours to be respected.
But Humanity, for some reason, cannot seem to grasp that one basic idea. And it applies to everything, not just religion. Science, politics,  sports, even video games suffer this problem we're all far too familiar with the Console Wars. Or when these guys into a fight over a Call of Duty match.
Really the only thing you could do to make the world a better place is to take away everyone's sense of opinion or free will, which would lead to no disagreements and thus no reason to fight or kill. But that's obviously a pipe dream, so we have the world we have.

Just because religion disappears overnight, doesn't mean the world would be a perfect place with people frolicking around the pastures and proclaiming the hills are singing to the sound of music. That's a common misconception lots of people have. Hunger will still exist, economic disparity and poverty will still exist, wars driven by other interests will still exist, abuse of all kinds will still exist, mean people with other interests will still exist... you get the idea. You remove a church and everything will be the same. Nordic countries are secular and they have a better lifestyle, but it doesn't mean the Bible Belt will turn good overnight if you remove churches. It's a matter of culture and the way of living in that area.

If it wasn't Islamic extremists it'd be something else

well yeah  people with power can do a lot of bad things, but again, that's not a religion thing, it's a human thing.

You have to give me an example there because to me it seems that your something else is just another religion.

1) what about those who found a reason to live from religion? Would they just be dead souls walking around?

2) there'll be issues regardless of religion(or lack of it). That's human society for you. Well never get along.

3) It's not necessarily religion that causes issues. It's the people who take it so seriously(i.e ISIS) that cause issues.

Doesn't need to be religion. It can be political ideologies, people in power, even terrorist organizations, etc.

No religion -> Judaism never existed -> Shabbat never existed -> concept of resting on the weekend never existed -> EVERYDAY IS A WORKDAY

Who would want to live in that kind of shitty world?

Religion isn't the problem. People use it as a shield to justify their actions. All they'd do is find something else to act in the name of. It wouldn't change a thing.

In this day and age at least, a world without religion would be preferable. But religion hasn't only had a negative impact on society historically (nor does it only have a negative impact on society today, but it's the net impact that's important).