Geese are borderline evil, one casually walked into my friend's house and then proceeded to attack everyone and destroy a bunch of things.
Shouldn't "gęsi" be in the genitive? After all, there is the word "nie," which implies that the sentence is negated.
Yes, and it is.
Also Genitive for "goose" and "geese" is identical so I added "a/the goose" as well.