Didnt said he takes down Shanks, i spoke about his clan.
My theory, which i stated in my Gunko thread, is that Shamlock was the one "killing"/taking Victoria, but since no one knew of a twin, specially not some children on a scum island, he must have assumed it was Shanks. He later saw shanks in the newspaper, looking identical, and was convinced it was him. Thats why he hunts Shanks. But originaly should and will hunt down Shamlock.