Complex sentence: He always hated Elendil and his sons because of their treachery: he believed they had betrayed Númenor by communicating with the Elves and remaining loyal to the "Valar".
Split into simpler sentences: He always hated Elendil and his sons because of their treachery. He believe they had betrayed Númenor by communicating with the Elves and being loyal to the "Valar".