Opening quote from official source
"2263 The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one’s own life; and the killing of the aggressor. . . . The one is intended, the other is not."65 "
vatican.va/archive/ccc_css/archive/catechism/p3s2c2a5.htm
Introducing the subject
I take no viewpoint here.
I will show the different degrees apparent, so as to show where the killing of others is permissable in general, by authority, also, self defense also is ligitimate reason to kill another within correct context, but I assume you understand that, and so, I will try to explain the answer.
The writings of Thomas Aquinas on “Murder” SSP Q 64
State his views on the destruction of what he terms a “Sinner” as regards the theology of his period, for the “betterment”, or preservation of a culture, or society.
"I answer that, As stated above (Article 1), it is lawful to kill dumb animals, in so far as they are naturally directed to man’s use, as the imperfect is directed to the perfect. Now every part is directed to the whole, as imperfect to perfect, wherefore every part is naturally for the sake of the whole. For this reason we observe that if the health of the whole body demands the excision of a member, through its being decayed or infectious to the other members, it will be both praiseworthy and advantageous to have it cut away. Now every individual person is compared to the whole community, as part to whole. Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good, since “a little leaven corrupteth the whole lump” (1 Corinthians 5:6). "
newadvent.org/summa/3064.htm#article2
However, he further states
"I answer that, As stated above (Article 2), it is lawful to kill an evildoer in so far as it is directed to the welfare of the whole community, so that it belongs to him alone who has charge of the community’s welfare. Thus it belongs to a physician to cut off a decayed limb, when he has been entrusted with the care of the health of the whole body. Now the care of the common good is entrusted to persons of rank having public authority: wherefore they alone, and not private individuals, can lawfully put evildoers to death. "
newadvent.org/summa/3064.htm#article3
Further, even the clerics cannot do such
"I answer that, It is unlawful for clerics to kill, for two reasons. First, because they are chosen for the ministry of the altar, whereon is represented the Passion of Christ slain “Who, when He was struck did not strike [Vulgate: ‘When He suffered, He threatened not’]” (1 Peter 2:23). Therefore it becomes not clerics to strike or kill: for ministers should imitate their master, according to Sirach 10:2, “As the judge of the people is himself, so also are his ministers.” The other reason is because clerics are entrusted with the ministry of the New Law, wherein no punishment of death or of bodily maiming is appointed: wherefore they should abstain from such things in order that they may be fitting ministers of the New Testament. "
(same site-
newadvent.org/summa/3064.htm#article6)
So, finally, it could be considered legitimate within context, or a foreign agent could perhaps legitimately do so, if there is danger to souls or lives, however, this is never left up to the individual, or only in some circumstances, whereby the church demands certain actions not usually be done, but as to these cases, when these persons become enemies of the systems they entered, e.g. democracy should protect people, Stalin got in by force, then there may be a case for force.
In either case, I by no means encourage such action, but from the top quote, it may be justified, given correct circumstance as far as the church stands, but this may differ in regards other views.
E.g. If such a person where to kill your friends, or persons, this may be exception to the rule, as seen above, but I will allow others to build on my research.
Of course, there could be other ways to achieve good results, but doubtful with these cases.
Conclusion
This is theoretical of course, and I am not in a position to freely comment on this matter, use the sources and others to decide on this theoretical moral dillema, as to “some ruthless” dictator, we must remember our our fallable state, and see that diplomacy is first attempted unless unachievable, then sanctions and other things, I still do not think this is for individuals to decide, according to doctrine, but I have not got the answer, I asked the same about the IRA, and was not answered, it is hard to determine the correct action.