My other question was why White Walkers were nearly naked in the first two seasons but wore clothes in armor in the later seasons.
And this is what user568458 said.
I'm not 100% sure but I think the first time we see white walkers wearing armour is also the first time we see them after Sam kills one with an obsidian dagger. I think before this, they were always sporting the "winter casual" skirts-and-bracers look. Maybe there was an off-screen "This s*** just got real. Put your obsidian-proof vests on, lads, and meet me by the baby-sacrificing table"
I never assumed the White Walkers knew Sam killed one of their brothers.
When Sam killed him, the White Walker was all alone. He wasn't traveling with his brothers, or else his brothers would have attacked Sam too. Since the walker was all alone, I don't think word got back to any of his brothers.
I also assumed the White Walkers did not know dragon glass or Valyrian steel could shatter them. Just look at the expression on the walker's face when Jon Snow blocked his attack with a Valyrian steel sword. That's when the Walkers realized "Shit just got real!"
Did they know Samwell Tarly killed one?
And a related question is whether they knew about their vulnerability to dragon glass and Valyrian steel.
I prefer answers from the books (with citations) over answers from the tv series.