Read
The Letters. Lots of gems of insight in there. Also, look up "mean" in the dictionary. I hardly think Tolkien meant Gollum simply stole candy from his little sister. Tolkien called him "mean and greedy" several times. I'm thinking he was leaning more toward the "characterized by petty selfishness or malice" and "below the normal standards of human decency and dignity" definitions. That sure sounds evil to me - which is also one to look up: "1 a: morally reprehensible : SINFUL, WICKED <an evil impulse> b: arising from actual or imputed bad character or conduct <a man of evil reputation>". There we have it. Evil Gollum.
He also called Déagol mean and greedy. Birds of a feather... to answer your "How would he even have a friend in the first place..." question.