Started playing Fallout 2. Love it. Got a quest, had to find Karl, blew it off. Accidentally let the Slag people get brutally killed, they were innocent. I feel awful now. How is this game so good???

You just explained how the game is good. There is no right or wrong way to do quests, but if you are very thorough, and attentive (to information and clues), you can better decide how you want to proceed. You clearly missed some clues here. That's what makes FO1/2 good.

This reminds me of something that could happen in the original Wasteland, resulting in a town becoming abandoned.

If you haven't played Wasteland, I recommend giving it a try; the Fallout series could be considered a spiritual successor to Wasteland.

Or when you dilly dally too much and by the time you meet the BoS they are too worried about the nearby army that they are too busy to give you any side quests and just send you straight on to the main missions. Or when you arrive at Necropolis late in the game while the muties are pouring across the map and all the friendly ghouls have been torn apart and left lying in piles.