The Incredibles

Public opinion turns against humans with superpowers â called "Supers" â and after facing several lawsuits over peripheral damage caused by their crime-fighting activities, the government forces them into civilian relocation programs. Fifteen years later, Bob and Helen Parr, formerly known as Mr. Incredible and Elastigirl, and their children Violet, Dash, and Jack-Jack live as a suburban family. Bob is dissatisfied with suburban life and his white-collar job and longs for the glory days. Sometimes, Bob and his old friend Lucius Best â formerly known as Frozone â perform vigilante work, without their wives' knowledge during the night.
One day, Bob loses his temper when his supervisor refuses to let him stop a mugging, causing him to lose his job. Returning home, Bob finds a message from a mysterious woman named Mirage, who convinces him to become Mr. Incredible again and gives him a mission to destroy a savage tripod-like robot called the Omnidroid on the remote island of Nomanisan, promising a substantial reward. Arriving on Nomanisan Island, Bob is able to find and destroy the Omnidroid by tricking it into ripping out its own power source.
Bob is rejuvenated by being able to use his powers freely, improving his attitude and relationship with his family, and he begins rigorous training while waiting for more work from Mirage. Discovering a tear in his suit, Bob visits superhero costume designer Edna Mode who decides to make him and his whole family suits, unbeknownst to Helen and the kids. Leaving for Nomanisan once again, Bob discovers that Mirage is working for Buddy Pine, a disaffected former super-fan rejected by Mr. Incredible who adopted the name Syndrome. Syndrome intends to perfect the Omnidroid and defeat it in public while manipulating its controls to become a hero himself, and then sell his inventions so everyone will become equally "super", making the term meaningless. Bob sneaks into Syndrome's island base and finds his computer. He discovers Syndrome has lured countless retired superheroes to their deaths, pitting them against previous Omnidroid prototypes to test their design. Meanwhile, Helen visits Edna, finds out what Bob has been up to, and activates a homing beacon Edna built into the suits to find him, inadvertently causing Bob to be discovered and captured.
Helen borrows an airplane to head for Nomanisan, but finds Violet and Dash have stowed away wearing their own suits, leaving Jack-Jack in the care of a babysitter. Syndrome picks up Helen's radio transmissions and shoots down the plane, but Helen and the kids survive and make it to the island, though Bob thinks they are dead. Bob threatens to kill Mirage to force Syndrome to release him, but Syndrome calls his bluff and Bob releases Mirage.
Helen proceeds to the base to find Bob, discovering Syndrome's intentions to send the Omnidroid to Metroville in a rocket. Distraught by Syndrome's behavior and his true plans, Mirage releases Bob and informs him that his family is alive. Helen arrives and races off with Bob to find their children. Dash and Violet use their powers to counter a number of Syndrome's guards in Nomanisan's tropical jungle before reuniting with their parents. The family is captured by Syndrome, who heads off to initiate his plan.
The Parrs escape and use a security RV and a spare orbital rocket system to travel to the danger in Metroville. True to its programming, the Omnidroid recognizes Syndrome as an opponent and attacks the remote on his wrist before knocking him out. The Parrs and Frozone team up to fight the robot in a losing battle until Bob comes across the remote. Using a fallen pincer of the robot, Elastigirl uses the remote to fire it at the Omnidroid's power core, destroying it. Returning home, the Parrs find Syndrome who plans to kidnap and raise Jack-Jack as his own sidekick to exact revenge on the family. As Syndrome tries to escape to his airplane, Jack-Jack's own shapeshifting superpowers start to manifest and escape Syndrome's grasp. Helen rescues Jack-Jack, and Bob kills Syndrome by throwing his own car at the plane which causes Syndrome to be sucked into the plane's turbine by his cape, causing it to explode.
Three months later, the Parrs, having adjusted to civilian life, witness the arrival of a new villain called the Underminer. The family dons their superhero outfits, preparing to face the new threat.

Question: Why does Syndrome plan to kidnap and raise Jack-Jack as his own sidekick?
Answer: Because he wants to have revenge on the family.