I might be late, but I'm thinking that samehada isnt really a sword. Maybe its more along the lines of a fragment of a tailed beast or is a tailed beast that just got sealed in a sword or something like that. Ans maaaaaybe....Kisame and Itachi didnt get along as much as we like.
Maybe madara put kisame with him so if itachi tried to run away he could just eat chakra or jutsus or something. I dunno. That part i dont really believe since it looked like they were actually friends, which leads me to believe hes not as bad as we think. But what ev, kishi will reveal the truth eventually.
If you're half the man I am today, you're twice the man you were yesterday.