Only...
in one scenario the blame seems reasonably placed whereas in the other it doesn't.
Well, that all comes down to opinion, doesn't it?
Well, tell me what your opinion is of these two scenarios:
Scenario A
You are a peaceful, hard-working, soon-to-be family man. While on a business trip, a wildfire starts in a forest near your house. A lone firefighter goes to stop the fire, but he is unable to do so. For some reason your wife is unable to escape, even though the fire doesn't really travel that fast, and she dies. You decide to find the firefighter, capture him, and make him watch as you burn his entire city, killing many innocent people just like your wife. He gets away and you find him 25 years later. Having had all that time to think about it, you're still bent on revenge and you carry out your plan. But there's a catch: You get offered a deal by a friendly angel to go back and save your city from burning and wife from dying. It won't fix the pain you had to feel, but it will seriously right a wrong. You refuse the angel's offer in favor of revenge over saving your loved one, and while your vengeance is carried out, you are killed in the process.
Scenario B
You are a cruel and power hungry dictator. Your country faces imminent defeat, so you set out to hide in a far away land with several companions in order to rebuild and one day return to your perceived rightful place as ruler. In your travels across the sea, a more powerful ship and its captain offers to bring your crew aboard when things don't fare well for your ship. In return for their generosity, you commandeer their ship and catch them off guard. After an ensuing struggle involving an attempt to kill the ship's captain, they manage to overthrow you, and they decide graciously not to kill you, but to strand you on a desert island with your companions. This island isn't so bad either. A few months later a tsunami hits the island killing your wife and several of your companions. Living conditions become really dire, but you manage to survive there for almost 20 years. Then one day a ship stops by the island, and you capture and torture a couple members of the crew to find out about the original captain who stranded you and a devastating new nuclear weapon they've found. You in turn strand all of their innocent crew and head off to find this weapon. You find the people making the weapon, who are mysteriously unguarded, and proceed to torture and kill them until they hand it over. They phone the military, and a ship arrives that is the very same one that stranded you many years ago. With your newly commandeered ship, you cripple the captain's ship with a surprise attack, but you aren't completely in the clear either. The captain outmaneuvers you and you are enraged by this. You could just let him go, but you don't want him to get the best of you and you pursue him. This arrogance allows you to be destroyed.
...
Those are the two options, and honestly B sounds a lot more reasonable to me, even if still unreasonable. Khan started as a madman, probably having to do with the genetic engineering. Someone sentences him to an unknowingly unsafe planet and as a result, his wife dies. Whereas Nero started off good, and never had someone put him in the line of danger. On top of that, Nero was given a chance to right the wrong... to revert to being the man he once was. Khan had no chance, and he was never really all that good to begin with.
