Human sacrifice & cannibalism never went away. We're on a farm. As voltaire said, to find who rules over you....

Way I see it; Jesus' death didn't mean the world got 'fixed' as in people still do horrible things to each other. What it means is that people who believe in Jesus Christ won't go to Hell. Fixing the world comes later.

At the same time, good sir, that same good book says something about "the truth will set you free" and "the only way for evil men to succeed is for good men to do nothing." "Faith without works is dead" and "you can tell a tree by it's fruits." There is something in the faith of Christianity that does call people to make the world a better place, even if so quietly-"to help orphans and widows in their distress without being corrupted by the world."
Now I'll level some criticism against myself, cuz there is a verse that says something like "it's shameful to even mention what the disobedient do in the dark." At the same time, concerning the Devil, I believe it was Paul who said "we are not unaware of his schemes."
I know the world will never be perfect, but people reap what they sow. Salt and light of the world. It can get better. It should get better, even if we screw up in the process of making it better.