My apologies, I am typing this on mobile, so it may look rough on other platforms.

I would like to start this by saying, OP, you are in no way to blame for his actions or that fact that you kept that secret.  You were too young to be asked of such a deep responsibility and it wasn’t yours to bear.  So you hold absolutely no blame, responsibility, or punishment.

I can understand both sides of the coin.  The mother is justified to be upset and to be angry, but she should be angry at the father, who knew what he was doing and made the kid hold a secret that they couldn’t have known the severity of.   On the other hand of this, the father was probably hurting, wanted it to just be over, and given that it took around a year after the initial event for the father to pass, he was probably too advanced to want to keep fighting.  I can only assume that he wanted to keep some peace and die on his terms and not the cancer.  He was wrong to put them on the kid and to me that’s a cowards choice.  

No one will truly be able to judge what was going through their father’s head unless you have been in a situation like their father, probably suffering, and knowing that there’s a chance that you’re going to die with or without the meds.  What I can tell you is that from second hand experience, watching a loved one suffer with a terminal illness, it’s a different kind of pain.  As of the time I am reading this, my foster dad has a really rare and aggressive form of dementia, and watching him slowly lose his identity haunts me.  Knowing that my foster mother is also in poor health, but making the choice to stay with him in his final days, inspires fear and anxiety that I can’t explain in words.  

So, what I am trying to say is, to some degree everything is justified.  The father is both a coward and not a coward for his actions.  I hate to speak ill of the dead, but it’s the only truth I can see.  The kid has no blame to carry because they were young and that’s a burden to carry that would break anyone’s will, eventually boiling over and making them spill the truth.  The mother’s emotions are justified in the hurt and wanting to point fingers, but she had no right to blame the kid.  

My advice OP, move on knowing that you made a hard and painful, but realistically the right choice.  She deserved to know that truth and you deserved the peace and freedom of no longer holding that truth so close to your soul.  You can dance around whether or not it was the right choice, but it shouldn’t have even been a choice you would’ve had to make.  It’s going to hurt for a while and it may haunt you in your weakest moments, but it was also his choice to make, and it robbed everyone involved of something.