Final answer:
The assassin's reasoning is an example of utilitarian ethical system where the belief is that the end justifies the means, controversially valuing overall outcomes over individual actions.
Step-by-step explanation:
The ethical system that the anti-abortionist assassin is using to justify their actions is known as utilitarianism. This philosophical viewpoint holds that the outcome that maximizes overall happiness or 'the greater good' is the most ethically correct choice, even if it means performing actions that are against conventional moral standards. Therefore, the reasoning that killing one person to save many lives upholds the principle of the end justifies the means, which is a hallmark of utilitarian thinking. However, this form of ethical reasoning is contentious and poses significant moral dilemmas, as it challenges the intrinsic value of individual human lives and the respect for legal and societal norms. It also leads to a debate on the sanctity of human life and the possible consequential risks of allowing individuals to take such drastic actions based on their moral calculus.