Loki Odinson was by no means a monster by physical or mental means, no, it was his title and his destiny which made him a monster.
For you see, when a child of Asgard is born, s/he is brought before the entire populace of Asgard in the great hall of the palace, where the most powerful of sorcerers show everyone the child's destiny, what their titles will be and what they shall take up as a trade. Sometimes it is good news, other times it is bad.
For Loki Odinson, "bad" was an understatement.
When Loki was brought before the populace of Asgard, many thought that he would be a great warrior, like his brother and father before him, and he was going to be, just not as they expected.
For what they saw was amazing, Loki was a great and powerful sorcerer and a magnificent warrior, his title was to be the God of Mischief, but there was something else in destiny, something dark, something terrible, a title. The title which no one wanted to bear, yet knew that someone had to;
The Bringer Of Ragnarok.
That was Loki's second title, the one which made him a monster.
There was outrage, as to be expected, for how do you cope when you find out a member of the Royal Family was going to destroy everything in the Nine Realms? That was why Loki never strayed far from the palace, if at all without Thor, anyone else wasn't enough to hold back the crowds of people who wanted him to burn, sometimes his guards joined in the chaos...
But Loki wasn't Asgard's greatest sorcerer for nothing, for what was a sorcerer without magic? He knew all about Asgard's secret passageways, his long stay in the palace taught him, he knew all the secret spells, the doorways to other realms, he knew them all, the books were his teachers, he was their pupil. So, using his immense supply of magic, Loki used it to locate someone, anyone, who was in his same predicament, It took a long while, but then he found someone. Full of excitement, he opened a passageway, and dashed through, he was off, to Midgard.