Behemoth is the codename given to a Nod supersoldier created through research performed on Siran Vardan. Unlike Siran, a forced subject, this soldier was a willing volunteer for the procedure.
Due to how he was created using research performed on Siran, and how she ran before she was 'finished' and 'betrayed' Nod, he views her as inferior to him and desires her utter annihilation.
However, his views here were proven erroneous, when, in 2289, Siran unlocked her Ascent Mode, "Hyper-Evolution Prototype", and literally obliterated Soldier Behemoth. Nothing remained of the supersoldier.