[Kamen Rider Exile]

I would have to agree with you that Otoya was a very sad death scene in Kiva, knowing that it would kill him he still chose to become Dark Kiva.

My personal saddest death in the series was probably the orphnoch trio from 555, both movie and series

Kiba- Movie: his last action to protect Mari really got me...as after all the tragedy's that happened to him, he still chose to go out with a bang...same reason for the series

Kaito & Yuka Movie: They were good hearted people...and especially when Kaito gets eaten by the Elasium Orphnoch...that was reallt disturbing.

Similar reason can be applied to the Roidmude trio