In the last 200 years Human  have left things for worse. What is wrong in leaving the world better place for your children through science?

I disagree with your premise. Yes, we have more pollution, but we are also living longer and have more toys. You wrote your question on a computer, didn't you?

Wars are probably the greatest harm.  They are full of killing and hatred.

Nothing at all.......the problem is, doing the right thing, is often a lot more expensive than doing the wrong thing. 

We all know dumping garbage in the ocean is terrible.....but we continue to do it, because it's the cheapest way of getting rid of it. 

Trouble is, much of it washes right back up on our shores, where it has to be dealt with a second time,   (and probably dumped right back into the ocean).............and  it's polluting the oceans beyond the pace the oceans can keep up. 

At some point, you would think all the brainacs of the world , would stop ocean dumping....but nahhhhhhh,  because it saves $$$$$$$$$$$$$$$$$$$$. 

but but but... then, people won't be able to get rich by exploitation!