One was a sin of omission (what Onan didn't do): he didn't impregnate Tamar. This was selfishness and disobedience.
The other was an act of commission (what he DID do): he perverted the act of intercourse by an intentional act of contraception.
Scripture says that "What he DID was evil in the sight of God, and so He killed him."
There is another provision known as halizah (Deuteronomy 25:9-10), which decrees that if a man refuses to carry out this duty, the woman must spit in his face -- in the presence of the town elders --- and remove one of his shoes. He is then to be called "this shoeless one'. This shows that a brother-in-law could opt out of Levirate marriage, and suffer no more than a public shaming.
In other words, mere failure to carry out the Levirate duty did not entail the dealth penalty --- far from it.
What aroused God's wrath was that other thing Onan did: performed a perverted act of contraception.
And that was how all of Christianity --- Catholic, Orthodox, and Protestant --- saw it until the Anglicans approved contraception at their decennial Lambeth Conference in 1930.
Was all of Christendom wrong for almost 2 millennia, until God vouchsafed to the Anglicans in 1930 that, really, He was OK with what Onan did?
I think that you make a good point and to be fair, the text may not be all that clear. Here’s another explanation for why the ‘death penalty’ was incurred in this instance. The shaming that you refer to as addressed in Deuteronomy 25: 9 and 10 is for the man who refuses to do his duty at all….He essentially makes it clear right up front that he wants to have no part of it. However, I think that we can all agree that scripture makes absolutely no provision for a brother to be boinking his sister-in-law or for that matter, anyone who is not one’s lawful wife with this one very narrow exception of Levirate marriage which comes into being after a husband’s death for the purpose of raising an heir. To be clear about it, that is not what Onan did….there is no record of him standing up and saying “I want no part of this Levirate marriage. I refuse to go along with this law”….In essence, what he did was far worse. He decided that he would make it LOOK like he was doing his duty by going along with the Levite marriage, he did in fact boink his sister-in-law but by then by pulling out at the last minute, he would still avoid the consequences of his sister-in-law getting pregnant. This is in fact a case where disobeying the law is a sin but making it look like you are obeying the law when all the while you are purposing to boink your sister-in-law with no consequences AND a scheme to circumvent the outcome, is a far worse sin. Onan was not killed for ‘performing a perverted act of contraception. He was killed for many many other reasons…. You could even say that adultery was one of the reasons. Why? Because since he had purposed in his heart to negate the express reason why the Levirate law was created, he was simply committing adultery with one exception…. Normally it takes two for adultery but in this case, it only took one since Tamar was innocent. You could even say that this was a form of rape since there is no record that Tamar agreed to have sex with her brother in law except for this one specific purpose of being impregnated as allowed by the law. What this story shows above all is how God views sex….. in a word, a holy act that has to be done his way. But this story is not a statement or lesson about contraception at all.
Here’s a similar case for you in scripture…. Acts 5: 1 to 10 is the story of Ananias and his wife Sapphira who sold a bunch of land. They gave the money to the apostles and claimed that they had given it all when in fact they kept some back for themselves. Both of them were killed. What was the sin? Was it the fact that they kept some back? Absolutely not…. if they had of gone to the apostles with the money and said right from the outset that “here is all we can give you… we need to keep some for ourselves for one reason or another”, everything would have been fine. The sin wasn’t in what Ananias and wife Sapphira did… it was in the pretences and sin that were involved with how they executed it.