Most people think they have developed their principles with reflection and consideration, but in my view most moral principles are post-facto rationalisations used to justify whatever-it-is the person wanted to do. So, excuses to justify the already decided upon action, rather than anything to determine the parameters of action, eg 'the money was too good', 'it smelled so good', etc.
Anyway, in answer to why people were callous back then and are so concerned now, I'd say nothing has changed, except what people view the norm to be. What seemed like 'callousness' was possibly considered 'practical' (or 'unsquemish'). For most moral relativists, whether they project 'practicality' or 'kindly concern' is simply an output of what they understand their social norms to be, rather than anything based in genuinely considered and applied principles.