If you remember, the whole Uchiha Clan was absent during attack. Which means, it's not necessary that he was dead. :yeah:He might have been killed in battle or something because when the kyuubi attacked everybody else in tobiramas group when he died was there except kagami and danzo
First - Kagami may have been an Uchiha that Tobirama trusted, but as he said they were only few like him. Which means, Kagami could have been abused/manipulated or something like that, by the Uchiha Clan, who relised that they were not treated right.
Second - Hiruzen is Hiruzen. He was raised with the Hokages and trained by them. It was kinda his destiny to become one himself.