This chapter is by GirlOfManyFandoms.
This theory was brought up by RainbowDashicorn.
Today, RainbowDashicorn commented a quite good theory. It is below:
Lil' Gideon Gleeful was after Mabel for some reason unknown. It couldn't have been love. He said something along the lines of, "I can't get your laugh out of my head," though the only time she laughed in front of him was while walking away from the Tent of Telepathy, but Gideon gave a more sinister look than a lovestruck one a boy gets from a beautiful girl's laugh. He had some other motive. I just don't know what...
Here is what I think.
Lil' Gideon really did love her, as revealed by the following:
He had a shrine of her.
Throughout the episode he referred to her as things like, "Soulmate," "Peach Dumpling'," and "Marshmallow."
While playing with a Mabel doll he created, he said in an "impression" of her, "Gideon, I still love you. If only my family weren't in the way."
But I think that his motive lies with a simple remark that most people only saw as foreshadowing of his evilness. While on the rooftop of a warehouse, Gideon says, "Mabel, when I'm up here lookin' down on all those little ol' people, I feel like I'm king of all I survey. I guess that makes you my queen."
My theory is that Gideon wants a queen. When he finally gets all the Journals he wants to use the Portal, which is obvious. And Portal, as we remember "...will open a gateway to infinite new worlds and herald a new era..." Simply, whomever is in charge of this Portal has a crapload of power, and can probably make you cringe at the power in his or her pinkie. Someone like that can definitely be considered powerful enough to be a king, and kings need queen.
So perhaps Gideon did love Mabel, but wants to rule the world with someone by his side. He seemed like a giddy fanboy when he was continuously petting Mabel's hair on the rooftop, and I don't know about you, but when I finally meet David Tennant in real life, I will pet his hair. I LOVE YOU AND YOUR GORGEOUS HAIR, DAVID TENNANT!