The premise of religion is that God defines morality. The weight of any action is determined by His standards. Because His Wisdom is greater than (an understatement) than man's wisdom and intellectual tools. So if anybody would know why one action is worse than another, it's Him.
I'm not an expert or authority, and I could be wrong. Here's my spiel:
From what I've learned about my faith (Islam), it's not the sin as much as it is the fact as it's disobedience towards Him. It's a complete lack of gratitude toward a God who has provided you with innumerable blessings. Redemption is not solely refraining from sin, but seeking to be close to Him and show gratitude. God can forgive sins where you harm or affect yourself (not praying, being greedy, neglecting your duties..etc) when you return to Him, be re-affirming His attributes and showing gratitude.
Then there are sins that you commit against others. God cannot forgive you until the person forgives you or is justified during this life or in the afterlife (Day of Accountability/Judgment).
Finally, there's disbelief....it's a transgression directly against God. A complete lack of gratitude. Ultimate disrespect. This is really between you and God to be honest.
There's judgment on Earth and there's His judgment. As humans, we only judge by what's apparent and so we can easily say "so and so is a disbeliever because his/her God has different attributes or he/she lacks faith" because the person make outward claims..etc.
Religion makes it apparent that no one knows what's in a person's heart. No matter how much you say so and so is going to Hell/Heaven. It's ultimately up to God to decide. Disbelievers are people who clearly know the truth but deny it. Again, no one knows what's in a person's heart/mind and how clear they see understand/view life. Although today, most of us would not fit into that category because everything is so fuzzy. We are limited by our access to accurate knowledge and the mere time it takes to sift through all the knowledge there is to make an accurate decision about reality.