I honestly think that at the beginging he wasn't evil. He was the 13 child from his family, so he was probably always forgotten/ not needed in a way. But, when he was Queen Elsa reveals her powers, and Princess Anna goes after her he got the idea of how to finally get power and be noticed/looked up to. So when Anna returned he tried to kill her so his plan would work. He played it of as Elsa killing her so Elsa would be killed and he could take power over the kingdom.