Pro-lifers do lose credibility if they tell a person to kill themselves, make death threats, blow up clinics, and/or other such actions. When they threaten another person's life, that is like killing an unborn child, but perhaps even worse. That person that they are threatening is a living being that has a future, and when they say those words, they lose what they stand for.
These people's actions don't speak for the entire pro-life position, so personally as a pro-choicer I don't discount pro-life arguments just because of the actions of the extremists. But it does greatly damage the public perception of them, and undercuts their position as the ones who care about human life.
I do think it's unfair to the pro-lifers who don't blow up clinics or tell people to kill themselves to group them with the people who do, because pro-life isn't an organization or a club, it's just a set of people who happen to hold the same belief on a particular topic. But that doesn't change the fact that many people will.