But, that's applying the standard of Heaven as being a club of human behaviors.
We're promised eternal life in Heaven, but that it will be a spiritual state of "all things" otherwise passing away, a place COMPLETELY DEVOID OF EVIL.
Our human state is fallen. We live in this earthly realm with evil, in a fallen environment. Evil such as this BTK enacted/allowed/proliferated has no place in Heaven and God doesn't describe eternal life as a continuance of our human suffering, this earthly realm, but of a place DEVOID OF EVIL. No more evil. No more suggestions or threats of evil. All God, all the time. All Good of God all the time.
Such that, Heaven is promised to us who God finds pleasing at judgement (longer story but I paraphrase here), that we will enter in spiritual form, renewed, with all that is not of God not accompanying us.
Such that, if this dreadful human is judged spiritually by God as having sincerely repented (again I paraphras), then who it will be in Heave will be devoid of evil inorder and as he exists in Heaven.
Thus, it won't be the "BTK killer" but his renewed and "edited" soul.
All the rest of us do know, spiritually and academically, is what we are told by God and Christ and what is said (by Christ) is that even if a man's sins be (as terrible as imaginable) they will pass away (under certain conditions, again I paraphrase), be forgiven (more paraphrasement here), because of Christ's gift on behalf of us all (he redeemed us all, all sin).
It's complicated and beyond most human understanding. God doesn't tell us to ignore or not punish humanly terrible sins, or even all sins, He just says that He has ways we cannot understand now, but will if and when we get where He is: heaven. By His terms, is the summation.
On the OTHER hand, God's no patsy, not "human" in the sense that He can't be fooled or manipulated and thus, others here try to point out the perfection of His judgement.
BTK can join his first female victim in Heaven, and she can look up and say, "Oh thank you, God, for allowing me to be slowly tortured to death in the spring of my life only to end up spending eternity with the monster who made me regret I was ever born."
It certainly is beyond my understanding.