On the 15th of June 2019, I answered the question "are there serial killers in Heaven?" for the Path Treader Ministries Facebook page. A follower of the page commented that this is one of the harder things to understand about God's grace. Why can He save someone as evil as a serial killer, but He is still willing to condemn a "better" person?
From an earthly perspective, I think we can all resonate with this. In fact, upon receiving Christ, penitent serial killers are usually the most humble individuals. Who deserves Heaven less than they do, right? So Jeffrey Dahmer, being the example I used in the original post, actually thought he was sinning just by being alive.
The way to think about it is that actually, there is nobody who deserves to go to Heaven, but also nobody who cannot go to Heaven. In our pride, we tend to compete to be "better" people than each other, and I do believe that we can be. You will find those who cite James 2:10 to basically say all sin is equal, but they're actually reading it wrong. It's not that all offences are equally bad, it's just that it's against the same God being offended. But ultimately, we have all rebelled against that same God, and so even the best of us deserve God's wrath. But it's up to us, in a sense, to decide whether or not we receive it.
Basically, when Jesus saves a serial killer, but condemns a simple liar, He isn't saying "I love this serial killer more", or "this serial killer is a better person than this liar". He's saying "this serial killer willingly received my righteousness, because I was punished for his sin, whereas this liar chose to rely on his own righteousness instead".
Think of it as being like a cruise ship that goes down. The ship has enough life boats that everyone on board can survive, but you have to actually get on one. If you're not on a life boat when the ship is under water, you're going to drown, or get eaten by sharks. Do the sharks care how much you paid to get on the ship? Does the ocean care if someone who did take a life boat is a better swimmer than you? Of course not; survival is about the boat, not the people in it. In the same way, salvation is about Jesus, not about us. In the end, as I like to say, there is no one in Heaven who isn't supposed to be there, and no one in Hell who is.