Carving out irrational and purely emotion-driven exceptions to the right to life makes the people who do it seem crazy and dangerous to me.
I wouldn't be able to pander to the irrational, ignorant, crazy and dangerous, no matter how well it paid.
In such cases, the only person who unequivocally committed no crime nor lack of judgement is the one being murdered, without recourse, defense, or due process.
The murder of defenseless innocents is "heartless", imho, not guaranteeing them a chance to live.
I fully agree that when we permit society to rationalize the murder of the most innocent, we are only a twist of logic from our own very personal extinction--and it is guaranteed that someone somewhere will find a reason, no matter how twisted.