There are different kinds of violence. Some violent acts are totally barred; some are justifiable. There are imporant factors at play:
- the reasons violence occurs (intent to kill or revenge vs. intent to stop violence),
- the levels of violence employed (proportional vs. excessive), and
- whether the violence is (a) aggressive violence or (b) responsive violence for a legitimate purpose.
Aggressive individual violence is always wrong. That is, attacking an innocent person (e.g., a person who is doing no harm to you, and no harm to anyone in your charge or even no harm to anyone else)...that is always wrong. This includes attacks against all innocent humans, including the unborn who are the most innocent of all humans. I understand how that last example can be hard for some people, but difficulty in compliance doesn't make it ok.
Defensive individual violence can be justifiable. If someone attacks you or attacks someone in your charge (e.g., your children), you can use appropriate responsive violence to stop the aggressor violence. Not only are you not required to stand by and allow someone to torture your child (you are justified to use violence to stop that), you may even be obligated to use violence to stop that.
Violence done as a subject-agent of authority (e.g., as a member of police or military) on behalf of a legitimate government is excused in many cases. In some cases (e.g., declared war in apparent compliance with international conventions) but not in all cases (e.g., genocides), the citizens acting may be excused from sinful responsibility when participating in violent behavior, but the authority leadership may be sinfully responsible for causing non-justifiable violence to occur.
The above is my simplification of a complex issue, and we can discuss nuances of each if you wish. Feel free to poke.
Regardless, outright pacifism is not required.
The Catholic Catechism walks through the issues very well. Check out the link.
Link: http://www.vatican.va/archive/ccc_css/archive/catechism/p3s2c2a5.htm