Background
Agaroth was created at the same time
Therion was, so they were deemed to be brothers.
Agaroth's purpose was unveiled to be the Valar Protector, whilst
Therion was meant to be the Lord of Harvest to replace
Melkor's old role he rejected.
Therion saw this too as a useless role, and fled the Timeless Halls. Since
Agaroth was close to
Therion, he would too flee.
Agaroth would slowly corrupt to evil, and it is possible there is still good in his soul somewhere. However, this cannot be checked as he has been vanquished by Titan.
Invasion of Almaren
Hearing of Alrea's whereabouts,
Agaroth would declare war on Almaren to defeat Alrea. Alrea would be vanquished, however
Agaroth too would die alongside her due to Titan.