So if you were looking for little treats to sprinkle into your next Astalor fic, maybe these can help:

Dislikes: stupid people, the outdoors, people who attack (too many) innocents/acquaintances, Death Knights, when enemies have a seemingly unbeatable power (that's his job), overly gory murder (of elves - memento mori perhaps), unreasonable foes, being hunted by an assassin (alone - he makes you defend him), beings he cannot look in the eye, water, drowning, the Void, curiosity, irredeemable research methods (indiscriminate killing), too many ambitious loners in Quel'thalas (he's the one doing things correctly, obviously).

Likes: when people are gushing with anguish, the city of Silvermoon, elven forces generally, beating enemies who act unstoppable, gardening, Farstriders (he keeps mentioning when they've been killed, and he regrets having to hunt one in particular), you (he warns you to be careful when Farstriders have already been murdered, and when you might get recycled into a pocket watch), ambition, talent, rumors.

Further thoughts:

Is he the one offering Lost Theldrin aid? Or it is just "aid"? Losing himself into an anguish crystal drop by drop might not be preferable to being hunted down and ended. But I bet Astalor doesn't think of it like that.

The Farstriders are getting wrecked by some of these guys. They're specifically named as dying in large numbers to a couple, in horrible ways. Halduron should be recruiting as hard as Rommath is! "Thin their numbers" shouldn't go both ways like this.

Astalor mentions having scouts. Unclear if they're his own dedicated scouts, or if he has lured in spies, perhaps among the Farstriders - which would account for him knowing of so many instances when they've fallen to these enemies. He doesn't like going outside, but these folks work outside all the time and are also dedicated to protecting Quel'thalas and its people. Why not employ them in a more direct manner? There are Farstriders all throughout the city, even when they're off-duty. Maybe some of them swing by the Murder Row Sanctum, or just pass messages. It's simply efficient to employ those comfortable in areas he isn't, after all, if he's going to cover all his bases in the defense of Silvermoon.

I like this headcanon. Seems reasonable.

If I were to expand into pure speculation, I'd be inclined to think that when Astalor called Ranger Swiftglade "one of our own," he meant it in the way that this had been one of his killers, who has lost himself to the hunt and become a danger himself. Much like some of the effects from Astalor's early experimentation with anguish gems and the like, anguish may simply have overtaken this Farstrider in particular, when all he intended to do was help keep Quel'thalas safe. Too bad we can't loot his corpse for anguish gems!