I could be misremembering but the Walking Dead season 1 and Game of Thrones season 1 came out around the same time, but didn’t really kill of any major characters until after the first season.

Both are based on pre-existing material but GoT source material predates the Walking Dead by 7 years.

Glen Cook was killing off major characters back in the 1980s.

... which started with "Night of the Living Dead"... :)