Sherlock knew what it was. He knew the definition. (Feelings of trust or affection felt in many cases of kidnapping or hostage-taking by a victim toward a captor.) He knew how often it happened. (In 27% of cases, approximately.) He knew historical contexts, and could probably recite the encyclopedia entry off by heart. (Why hadn't he deleted that? Useless. Make a note.)
But a definition didn't mean anything on its own. Definitions were useless, abstract. Statistics meant nothing without context. Facts alone meant nothing. (What was that quote he had once heard? Shakespeare perhaps? Words without thoughts never to heaven go. Relevant? No, not really. Sentiment.)
But this was real. Now. Happening. He could connect that definition to a real life example. (Had always wanted them to do that during school, pointless, he thought. Now, unsure.) Context. Situational proof.
And to be honest, it sucked.
He much preferred keep things compartmentalized, emotions separate from facts. (He does have emotions. Rather irritating things most of the time, but what can he do?) This is only making it difficult to think clearly, rationally.
John knew he has feelings. Never told anyone else about this. Never really needed to. They didn't talk about it. Sherlock snapped at Anderson, told him he was a sociopath, not a psychopath. It wasn't true. (E, none of the above.) But Anderson was stupid and it made him shut up.
So he clung to it.
John had seen him feel. (Mrs Hudson's been attacked by an American. I'm restoring balance to the universe. Look at me. I'm afraid, John. Afraid. What are you so happy about?)
It happens. Sentiment. (Sentiment is a chemical defect found in the losing side.) He greatly dislikes it. Clouds his judgement. Make him do things he would otherwise. Makes him care.
So. Sentiment, not such a good thing when you're being held hostage. Perhaps fear for your own safety, or for the safety of someone you love. (Love is a much more vicious motivator.) But this was neither.
What was this? Oh... pity.
Is it? Why is it?
Or perhaps it could be... no. Not that. John had explained that to him many times, the concept of empathy, and Sherlock still couldn't quite grasp it. The definition he could handle (of course, simple) but it always went back to meaning in context. And that was, invariably, where he got stuck.
But this could be it.
Because even though this man had kidnapped him, beaten him, tortured him, threatened him, threatened John's safety (bad idea, so very bad), and left him tied to a chair for days (was it days, time seemed different here) Sherlock didn't hate him. He knew he should. But he didn't. Couldn't?
Because Sherlock had seen this man's life story. Had known all the pain he suffered. (Small old burns on hands, likely from cigarettes, noting his age and the age of the scars, inflicted from age four until 16, likely when he ran away from home. Abusive father, or perhaps stepfather, need more data. Broken nose at least twice, never set properly. Multiple fractures that were seen in abuse victims as a result of holding their arms up to shield themselves, never properly set. Mental state: broken, tormented, angry, lashing out. He could go on, but does it matter?) He saw the pain. He even, irrationally, could feel his pain.
He now had context. He now had situational proof.
When John came storming in with Lestrade and half of Scotland Yard a couple of hours after Sherlock had reached this conclusion, and the man pulled a knife (not actually a threat, bluffing, bluffing, obvious) John shot him.
No one would blame him. He was terrified for his friend's safety, the man looked deranged, he appeared to be a threat. But Sherlock had seen that pain and the resulting thought process that had gone into the decision to pick up that knife. He understood it.
He let John pick up his pieces. He let him take him to the hospital ("if it makes you feel better, I really am fine though"). He let John explain the concept of empathy once more, and finally understood it.