It wasn’t brutal- kids back then were watching Wild Kingdom lions and hyenas and such, and going hunting after school; the cartoon version of Watership Down was nothing in comparison. This was when animals were your property and not “fur babies.”
Most kids weren’t going hunting after school in 1978. Watership Down is a brutal story, especially for a 1978 animation. It’s good, but it definitely earned that PG, in an era where almost all animation was G.