What do protestants make of this from 1 John 5?
16If anyone sees his brother commit a sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that. 17All wrongdoing is sin, and there is sin that does not lead to death.
I see that you are RCC and I think this probably has to do with mortal and venial sins. If I recall correctly, RC teaching when I was young held that murder, adultery, rape and other heinous sins were mortal and if you died without confessing them and getting forgiveness, you went to hell. Petty theft, "white lies," and the like were venial sins and if you died with only them unforgiven, you went to Purgatory, until your sins were purged by fire, and then you could enter heaven.
I reject that dichotomy as anti-biblical.
A couple years after I was saved, my mother asked me to pray for one of their friends who was taken seriously ill. As I started to pray, the Holy Spirit said strongly in me and in no uncertain terms, "Do not pray to me for that man!" I was stunned, but immediately remembered the verse you cited in I John. I asked a friend whose spiritual understanding far exceeded my own. He agreed with my understanding after I told him what sort of man he was.
A popular understanding of the unforgivable sin as blasphemy of the Holy Spirit says that if you die unrepentant, you have rejected the grace of God for your entire lifetime. Those who hold this view tend to be free-willers. I am not one. It is clear to me that this man blasphemed the Holy Spirit earlier in his life and he lived for many years after the above described event. I am not sure what that blasphemy entailed, and God has not since told me not to pray for anyone else.
As for some sins being worse than others, it is clear in Scripture that this is so and on many levels. For starters, take that OT penalties varied according to the degree of sin. Further, God makes distinctions among "sin, transgression, and iniquity." Stealing to feed your hungry family was less serious than stealing for greed. Both were wrong and required penalties, but those penalties differed.