Sengoku painted him as worse than Roger or Luffy, because he was killed in battle by Garp and Roger.
There was a buster call and everything, but there were normal people on the island. Of course there was Ivankov and Kuma there, there were some kids.
The treasure may or may not be Shakky because Xebec isn’t interested anymore, but Roger battling him over Shakky’s treasure is possible (for Roger’s interest).