It may seem obvious that Gantu managing to capture several experiments builds up rage inside Metallix, but why does Metallix seem calm when Gantu captured Tank, Hunkahunka, Fibber, Nosy, Amnesio and Poxy, yet when Angel is captured, he goes in Unstoppable Rage mode? Well, consider this: in the original film, Lilo spends so much time with Stitch trying to help him become a better person only for nothing to improve for her and him (or so it seemed before the climax hit), she runs out of patience with him, telling him to go away after he accidentally destroys her house in his fight with Jumba. If you think about it, Angel is the only one of the experiments that Gantu captured who managed to improve as a person, despite the fact Angel only improved as a person because she realizes she loves Stitch. Metallix didn't go berserk because Gantu was ruining his ship; he went berserk because Gantu tried to make Angel's redemption be for nothing.
It's rather odd that Metallix claims that Lilo's actual worst fear is cars and rain, despite her claim that she fears broccoli, clowns and the nearby haunted mansion. However, keep in mind that Lilo's parents died in a car crash on a rainy day. Hence why Metallix was shocked when Spooky transformed into a clown instead of a car when he faced Lilo.
Speaking of Spooky, some of his transformations are genius on his part:
When faced by Metallix, Spooky transformed into a wounded Tails, asking him "Sonic... why?". Metallix actually is the roboticized future self of Sonic the Hedgehog, thing which only Stitch knew when Spooky was activated. Combined with Fridge Horror (see below).
When facing Pleakley, Spooky transformed in Pleakley's mother. While at first seems to be Fridge Horror, it's turned into Fridge Brilliance by Fibber: you can judge by the way Pleakley's mother speaks that Pleakley's worst fear was disappointing his mother, which actually is a common fear among "Well Done, Son!" Guys.
Also, let's not forget Spooky transforming into Cobra Bubbles to threaten Nani to take Lilo away.
Why did Rule of Romantic make everyone forget that Stitch was supposed to upload the anti-Hamsterviel program into Angel? Because, as it turns out, some experiments were already fated to be romantic companions, the first encountered example being Yin and Yang. In Stitch's case, the program was activated by Terra, who told Stitch that friendship is something you feel. Since Word of God stated that, at least in his series, Terra has romantic feelings for Aqua and cares for Ventus like a father cares for their child, it can be said that true friendship and pure love were the catalyst required to activate the anti-Hamsterviel program.
Why did Metallix react to 627 with Tai Lung's most famous quote? Because, according to Dr. Koopa, he was created to destroy abominations, and 627 is pretty much one.
Why Stitch is the only one who knows that Metallix is the roboticized future self of Sonic the Hedgehog? Metallix should know he can trust Dr. Koopa. Except, of course, Dr. Koopa gave him so much power, Metallix's worst nightmare became this power. But why Spooky transforming into Tails? It's because Metallix has failed Tails before, when he was roboticized in the first place, and knowing he was sent to the past, he fears he'd let all of his friends down again. He only brought himself to tell Stitch because Metallix was projecting himself into Stitch, since Stitch is a living Doomsday Device, and Dr. Koopa gave Metallix the power of being a Doomsday Device.
This makes Metallix going into Unstoppable Rage mode after Angel is kidnapped all the more tragic: Metallix felt that Stitch was feeling he failed Angel, and, with Metallix not only projecting himself into Stitch, but also technically being part of Stitch's 'ohana, Metallix's worst nightmare came to be. Metallix went berserk because he didn't want his backstory to apply to Stitch.
