Lee surrendered on April the 9th. Lincoln was assassinated on April 14th.
Much of the leniency for southern leaders can be blamed on Andrew Johnson, lincon's VP. He was a democrat, known to support states' rights, granted amnesty to southern leaders, and did absolutely nothing to help newly freed slaves.
Once slavery was "absolished" in the south, due to Johnson's lack of empathy and concern, slavery as essentially reinstated. Blacks who didn't know how to read and didn't have anything saved up couldn't buy land were left with limited opportunities.
Whites unpunished for treason weren't going to obey the law because it was signed on a piece of paper. So quickly, blacks fell back into slavery, whites became emboldened, and in subsequent decades the Jim Crow laws were set up and statues were built.
And the cycle continues to this day. Newly freed slaves were left behind by the government and their descendants have yet to catch up. Many remained in poverty, lacking good educational opportunities, which has led to decreased employment opportunities. Confederate leaders, unpunished, were allowed to set up systems of inequality that have yet to be eliminated today. Their ancestors have maintained discrimination, and these statues are part of it. To this day, Harvard graduates who are black, earn less than those who are white and see fewer job opportunities. Harvard folks, Harvard.
I have a few family members who ask why so many blacks are poor and why they complain about discrimination. It's because it was never addressed after the civil war.
Had republican abolitionists been able to push through their agenda fully, blacks would've been given land and money (resettlement stipends essentially), confederate leaders would've been punished, and today we'd probably see less inequality and confederate statues.