Star Wars’ original bounty hunter got an early start on his training. Attack of the Clones showed that Boba Fett was an unaltered Clone of Jango Fett and that he went with his father on many missions in his early years. Even as a child, Boba was skilled, callous and willing to kill. And that meant when he saw his father die at the hands of a Jedi, it gave him a desire for revenge that couldn't be easily satisfied. Thus, Boba chose the legendary Jedi Master Windu as his first target. And while he failed to get his revenge on Windu, the young bounty hunter did gain his first on-screen kill as a result of his efforts.
The story of Boba's first on-screen kill is told in Star Wars: The Clone Wars, Season 2, Episode 20, “Death Trap.” With the help of Aurra Sing and Bossk, Boba infiltrated a squad of Clone cadets under the codename “Lucky.” It was a solid plan because the squad was scheduled to meet and train on the Jedi Cruiser Endurance, which was commanded by none other than Mace Windu.
As a cadet, he impressed his superiors, so much so that Admiral Killian told the other cadets that “training is no match for experience.” The group was attempting to shoot down malfunctioning droid parts as part of a training exercise, and Boba was the only one that could hit anything. Clearly, traveling the galaxy with Jango had given him the experiences that put him ahead of others around him.
Later in the episode, as the cadets were marching through a corridor, Boba finally put his plot into action. He snuck away from his squad and planted an explosive in the entryway of Windu’s quarters, getting back before anyone could notice he had been missing. And his plan almost worked to perfection when Windu nearly set off the trip-laser that would've activated the explosive.
Fortunately for Windu, as he stepped into his quarters, a Clone informed him that Anakin Skywalker requested his immediate presence on the bridge. Seeing the Clone’s urgency, Windu didn’t enter his quarters, opting to instead ask his subordinate to put something away for him. Following orders, the Clone did as he was told and entered the room, which set of the large explosion and killed him.
Boba was angry when he learned that Windu had somehow survived, which only caused his lust for revenge to burn hotter. He would go on to blow up the ship’s reactor core after Aurra Sing instructed him to because there was a chance that Windu would be killed as the ship went down. That, of course, failed as well, and Boba would have to continue his quest for revenge in the following episode.
But the fact that a random Clone was Boba’s first on-screen kill is kind of poetic. Most of the other cadets treated him well, but he clearly never fit in with them. Boba never truly considered himself a Clone, which was made evident when he told another cadet, “You’re not my brother,” as he stunned him with a blaster. At the time, he wanted nothing more than to kill Windu, and that drive would serve him well throughout his bounty hunting career.