I haven't encountered that specific event, but yes, you'll often run into random NPC instances while exploring/questing. And no, they're not important or quest-related. I've commonly seen the one where two NPCs are arguing over a unlit campfire; I've approached the campfire, lit it for them, they thank me and sit down, and I'm rewarded with a temporary boost to my stamina pool. I've also seen the instances (all of which you can participate in/intervene) of mercenaries vs. traveling traders, mages summoning Daedra, a Wood Elf threatening a Khajiit for breaking the Green Pact, NPCs setting you up to be ambushed by pretending to need help, Cryomage vs. Pyromage showdown, etc. It's random and the outcome/your decision to participate has no impact on anything important.

The senches were probably what "killed his family", and he was likely scripted to die anyway. I don't see why any "important" NPC would be randomly killed like that. I'll have to look for that one next time I'm in Malabal Tor.