Here is one good article on how being authentically Pro-life means we cannot use violence to stop abortion. Its a rationalist and not a religious argument and that makes it interesting and helpful when speaking to non-religious people.
THE FALLACY OF JUSTIFIABLE HOMICIDE
A Pro-Life Progressive’s Response to the Tiller Murder
4 June 2009
by Jen Roth
“Justifiable homicide,” they call it.
On May 31, George Tiller was shot and killed in the narthex of his church in Wichita,
Kansas. Tiller was best known for operating an abortion clinic that performed second-
and third-trimester abortions. Scott Roeder, a former member of the Montana Freemen
and an associate of anti-abortion extremists, has been charged with Tiller’s murder.