Who killed that noble? Probably some assassin hired by another noble.
"Noble families were the bulwark of power during these times. Assassination of a powerful house leader was seen as a critical blow to those families' fortunes. With the rise of religious strife assassinations of rival religion's nobles became increasingly common."
Crush would mean a punishment to crush aristocratic opposition, I don't think you mean peasants or burghers.
It doesn't say who actually arranged the assassination. It could have been a noble or a peasant/burgher.
Whichever it was, their action resulted in a weakening of a noble families' power. i.e. it would decrease the overall power of the Aristocracy.
Since the assassins "opposition" is to the power of a noble family, crushing them is surely going to be done by the king/nobility, to restore that power.
If so, that means a move back towards Aristocracy (restoring the power of the nobility, actually going even further and increasing it).
In truth, the wording of the description assumes that the state is Aristocratic biased. Otherwise it makes little sense. And that is a further reason why i suggest that crushing the opposition would mean a move further in that direction.
Though thinking about it again, having written this, I think it would make more sense to move the "Aristocracy -1" from option b) to option a), rather than change it to a +1 in option b). What do you think?