Hiddles is the perfect Loki because he works so hard at knowing him. He doesn't just "play Loki," he absolutely understands him. There are things that only Loki and Tom know about the character; Hidden aspects of Loki's hurting, pain, and therefore, his humanity.
"I had to find his smile, his innate love of anarchy and chaos. I had to make his pain real. Great villains are born from tragic circumstances. They become dangerous when their vulnerability hardens into vengeance." [Tom Hiddleston on Loki]. Source: http://www.tv3.ie/xpose/article/entertainment-news/171076/Tom-Hiddleston-lucky-to-play-Loki