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The Vatican recently supports the use of military force against ISIS, as noted in this article.
On that same note, per the example of your post, such a thing (be the person bad or not) would be murder. As such, the Vatican view on such a thing falls under the Fifth Commandment - "Thou shall not kill." Particularly, since you specify an independent, section 2265.
EDIT: To justify my answer, I'll also provide some Hebrew.
In the case of killing in the Ten Commandments (Exodus 20:1-17) we are given "Χ¨Χ¦Χ" (retzach) which translates "to break, to dash to pieces" as well as "to slay, kill, murder" (Strong's Concordance H7523). This is brought up again in Deuteronomy 5:4-21 reiteration of the Ten Commandments.
All cases of the 10 Commandments, simply put, are "Thou shall not kill. Simply put, no exceptions. According to the Priestly Code of the Book of Numbers, killing anyone outside the context of war with a weapon, or in unarmed combat, is considered "retzach". It should be noted that the Bible never uses "retzach" in connection with war.
There are a few instances where killing another is justified in Scripture - such as in legitimate warfare, capital punishment and legitimate self defense. However, Jesus Christ taught us:
You have heard that it was said, 'An eye for an eye, and a tooth for a tooth.' But I tell you, do not resist an evil person. If someone strikes you on the right cheek, turn to him the other also. (Matt. 5:38-39)
Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. (Matt. 5:43-48, Luke 6:27-28)
Put your sword back in its place...for all who draw the sword will die by the sword. (Matt. 26:52)
Blessed are the peacemakers, for they shall be called sons of God. (Matt. 5:9)
The above seem to indicate pacifism. With these taken into account, in the context of your question, an independent Catholic going out to kill the leader of ISIS would be viewed as a murderer - because s/he was neither defending themselves or fighting in war, legitimately. Nor would such a person be carrying out a capital punishment.