For the simple reason that I feel that the pro-life movement speaks for me in some ways, and I would hate to have that voice marginalized by the actions of a murderer.
Then don't let the pro-life movement speak for you -- go out and speak for yourself, and don't ever feel a need to apologize for the actions of people who happen to have the same opinion as you on a particular subject.
I suspect Hill's case has evoked a lot of intense debate in the pro-life movement primarily because what has become known as the "mainstream" pro-life movement is having a difficult time facing the brutal reality of what it is trying to deal with.