I think there are multiple factors and answers to this question. First off, by choosing to CTB, it goes against the just world and rosy tinted perception of life that most pro-lifers have, which means that they have to face the truth that they don't want to face, hence, just by that alone, they have a knee-jerk reaction to CTB itself. It scares them that they must face their (inevitable) mortality and reality, and it shocks them to the core that their emotions supersede their reasoning. Then, it is jealousy and envy (which also includes anger somewhere along their emotions) that they could not overcome their own innate self-preservation (or SI, the survival instinct) biological programming, thus they lash out irrationally. Finally, the indoctrination and programming, upbringing of most individuals by institutions, society, family, and their peers that CTB is wrong and life is always worth living (which oftenly is not the case!), thus when that belief and tenet is threatened, they behave irrationally (goes back to the first reason).
I'm not a psychologist nor doctor, but these are just my thoughts and understanding of why people insult the ones who have CTB'd.