Dearest Reporter... peace to you!
Please know that I did not take exception with your original post. As I stated, it made me a bit uneasy (I am not sure why, other than perhaps the 'truths' it contained). But your subsequent post was, well, uncalled for... IMHO. There are all kinds of people here who chide and ridicule. But so what? Isn't that to be expected of a "faithless generation"... no matter what truth or philosophy one is trying to get across? Sorry, but methinks you blew it... and you're not the first person (and surely not the last) that I will say that truth to. And it has been said to me as well... for as you stated, you... and I... are flesh-and-blood. Which brings me to your question:
Where does "man's wicked heart" come from?
It "originates" with the flesh we inherited from Adam... which is defective... and the "battle" between that flesh... and the spirit IN it... which are "at enmity".
Where's the evidence?
Good lordy... look around you!
Because we now exist in the physical world, the physical man tends to have more frequent dominion over the spiritual man. Some give themselves completely over to the flesh. Others are as spoken about by Paul (?), that when they wish to do good, but end up doing bad, it is not them, but the SIN that is IN them... in the flesh. Like Peter, who denied Christ. He truly didn't WANT to... and THOUGHT he was "standing". My Lord, however, knew what was "in" him... and told him.
That sin... inherent in the physical man ("skin in behalf of skin and all that a man has he will give in behalf of his soul")... is sometimes overpowering and we are lost to "win" against it. It is for that VERY reason... that we NEED... a ransom: we CANNOT keep the Law perfectly, though there are many, particularly the WTBTS... who attempt to mislead you into thinking that you can. But if you COULD, what need would you have of a saviour? You could save yourself, yes, by means of having kept the Law perfectly? But who of us can?
So, a NEW law was arranged, with PERFECT blood, which law says that if we live by IT, the law of LOVE... we can have our sins "covered". That is the provision of the "New" Covenant. And no, we cannot simply BELIEVE... we have to EXERCISE that belief... that faith... by doing the "works" of the New Law... works befitting the "fruit" of God's spirit... works that DEMONSTRATE our faith... works of love. (Isaiah 58:6-14)
And please know that I have not "studied" Christianity intensely; rather, I have been taught... by holy spirit.
I hope this helps, and again, peace to you.
Your servant and a slave of Christ,
SJ