No-one really needs who they Evil Leapers are or where they came from. However, that does not stop us from having plenty of theories
In the episode immediately before
Deliver Us From Evil, Sam is drugged by some shady members of the US Government working on Alien Lifeforms Forms. Whilst being drugged, Sam revealed all about himself and all the clearance codes for Project Quantum Leap. Is is a coincidence that the Evil Leapers turned up the very next episode? I think not.
However, I think that the Evil Leapers have gone beyond the control now of the shady US Government officials and is set well beyond Sam and Al's future. Could it be Satan? Well that depends if Al the Bartender really is God, but I doubt it. Satan knows that Sam exists from
B**gieman, but the evil leapers never heard of it and neither did Lothas.
Is Alia the exact opposite of Sam? No. Why, well she came good in the end. If she was Sam's opposite, I don't believe she would have. I do, however believe that Zoey is Sam's exact opposite and that Thames is Al's.
As to Alia and Zoey's link to Evil Project Quantum Leap, well that is something I guess we will never know.