How does the special ability of the Kill Marine work with other squad modes? The kill team in squad mode initiates a codex attack pattern not on the Kill marines list, can he take advantage of that mode or is he restricted only to using his cohesion and his squad modes? Also, how do squad modes work if the Kill Marine is the squad leader?
A Kill-Marine is still a space marine and benefits from squad mode abilities in the same manner as non Kill-Marines.
They may join squad mode with the other marines in their units following the normal rules for squad mode (they must take the test since it is not only themselves participating in the squad mode, it takes 2 to tango as it were). If they do so, they may use and take advantage of any squad mode abilities they know or have available from their oath.
The Kill-Marine knows all the codex and chapter modes a normal marine of his chapter and speciality would know.
The only difference is a Kill-Marine may utilize specific squad mode abilities without being in a typical squad mode.
A Kill-Marine can lead a Kill-Team just as any other marine would. The team's cohesion being determined normally.
The two types of cohesion (the team's and the Kill-Marine's) posses an issue. But, its not that difficult. The Kill-marine generates his cohesion and the kill-team generates theirs. The Kill-Marine's personal cohesion pool does not benefit from any special rules or items from other marines which increase cohesion (chaplain or blood of heroes for instance), nor can another marine's special ability or fate point be used to regen the Kill-Marine's cohesion pool. If a Kill-Marine is in squad mode when any normal squad mode abilities are used the cohesion cost of that ability affects both the team's cohesion and the Kill-Marine's personal pool. If the Kill-Marine is in solo mode when the team uses a Squad Mode ability the Kill-Marine's pool is unaffected.