Both, the sinner chooses Hell and God sends them there, but interestingly the Catechism (of Trent, by the way. The recent ones are pozzed imo) emphasizes the just deserts of the sinner:
>Those who depart this life sometimes leave behind them children who imitate their conduct, dependents, followers and others who admire and advocate their example, language and actions. Now by all these circumstances the rewards or punishments of the dead must needs be increased, since the good or bad influence of example, affecting as it does the conduct of many, is to terminate only with the end of the world. Justice demands that in order to form a proper estimate of all these good or bad actions and words a thorough investigation should be made. This, however, could not be without a general judgment of all men.
>Moreover, as the character of the virtuous frequently suffers from misrepresentation, while that of the wicked obtains the commendation of virtue, the justice of God demands that the former recover, in the public assembly and judgment of all men, the good name of which they had been unjustly deprived before men.
>Again, as the just and the wicked performed their good and evil actions in this life not without the cooperation of the body, it necessarily follows that these actions belong also to the body as to their instrument. It was, therefore, altogether suitable that the body should share with the soul the due rewards of eternal glory or punishment. But this can only be accomplished by means of a general resurrection and of a general judgment.
>Next, it is important to prove that in prosperity and adversity, which are sometimes the promiscuous lot of the good and of the bad, everything is done and ordered by an all-wise and all-just Providence. It was, therefore, necessary not only that rewards should await the just and punishments the wicked, in the life to come, but that they should be awarded by a public and general judgment. Thus they will become better known and will be rendered more conspicuous to all; and in atonement for the unwarranted murmurings, to which on seeing the wicked abound in wealth and flourish in honours even the Saints themselves, as men, have sometimes given expression, a tribute of praise will be offered by all to the justice and Providence of God. "My feet," says the Prophet, "were almost moved, my steps had well nigh slipped, because I had a zeal on occasion of the wicked, seeing the prosperity of sinners; and a little after: Behold! these are sinners and yet abounding in the world, they have obtained riches; and I said, Then have I in vain justified my heart, and washed my hands among the innocent; and I have been scourged all the day, and my chastisement hath been in the morning." This has been the frequent complaint of many, and a general judgment is therefore necessary, lest perhaps men may be tempted to say that God walketh about the poles of heaven, and regards not the earth.
http://www.catholicapologetics.info/thechurch/catechism/ApostlesCreed07.shtml
If you would like to explore this subject in more detail, I highly recommend the following links:
http://edwardfeser.blogspot.com/2016/11/does-god-damn-you.html
http://edwardfeser.blogspot.com/2016/10/how-to-go-to-hell_29.html
http://edwardfeser.blogspot.com/2016/12/why-not-annihilation.html