What does the damage shorthand in Nethack mean

If you have a stethoscope or wand of probing you can use it on the monster. If it tells you that the monster is scared, attacking it would violate your code of conduct.

Without these items there's no way to know for sure other than observing the monster for a couple of turns and see whether it looks like it's fleeing (note that the monster attacking you does not necessarily mean it is not scared, which is kind of unfair, but such is life).

That being said the penalties for these violations aren't too severe, so unless you attack fleeing monsters a lot, you don't need to worry about it.

Crawl Stone Soup is generally regarded to be harder than Nethack. The resistances are harder to acquire (eating corpses won't grant them) and, although each level is much larger than Nethack's, monster regeneration is much slower — once you've killed all that there was to kill, the level will remain mostly empty for a long while and you'll have to move on.

Crawl, however, is much more "ergonomic", with all kinds of fancy features like autoexplore or "search for items matching this regexp". It also features a richer religion system, a richer randart system, more interesting (and dangerous) dungeon features, a superior tiled interface and a tutorial. It features less complex item interaction, however: no dipping or scratching or applying or wiping; only wearable items can be cursed (IIRC) and there's no "blessed" status at all; throwing potions at monsters is ineffective; etc.

If you want to give Crawl a try, go for a Spriggan Enchanter, train stabbing and stealth and learn Ensorcelled hibernation and Confusion. That's good enough to explore a few branches of the dungeons — once you realize you heave really few hitpoints and they ain't regenerating quickly either (I mean, you're no troll).