Suder suffered from unusually strong violent tendencies for most of his life. He had tried a number of treatment options for his condition including violent holographic programs and targeted synaptic therapy. Neither of these were successful, so he did his best to control himself on his own. (VOY: "Meld")
In 2370, he saw an opportunity to provide an outlet for his urges by joining the Maquis, even though his homeworld was not under threat. He worked under Chakotay and was an effective member of his cell, although he did not engender much affection from his colleagues. They regarded him as too efficient when killing Cardassians and this made them uncomfortable around him. In fact, Chakotay had to hold him back in excessive situations, an action that almost caused himself harm. (VOY: "Meld")
He was with Chakotay's team on the Val Jean in 2371 when his ship became stranded in the Delta Quadrant. He later joined the crew of Voyager, which was also stranded there, despite his dislike of Starfleet. (cf. VOY: "Caretaker")
In 2372, after spending the next ten months without an outlet for his violent tendencies, he murdered Crewman Frank Darwin with a coil spanner and hid the body. He had no motivation for killing him, and did not know him. A brief investigation by Tuvok uncovered his crime, to which Suder freely admitted, but Tuvok wanted to have a more convincing motive.
Suder agreed to a mind meld with him, with the aim of helping him to control his impulses. While this proved to be somewhat effective for Suder, the resulting trauma to the Vulcan's emotional systems resulted in him losing some of his own control, becoming dangerous and violent. Tuvok tried to execute Suder for his crime, an action which Suder genuinely agreed with, but he was not able to let himself do it due to the treatment he was receiving. Instead, Tuvok collapsed and Suder instead alerted the crew.
Over the next few months, Suder worked with Tuvok to learn how to maintain his self-control. Suder began to find enjoyment in other activities, and under Tuvok's guidance, he took an interest in floriculture, and began to breeding orchids, and eventually became proficient at genetic manipulation of plants. He wanted to use these talents to contribute to the ship in a positive way by increasing the output of the ship's airponics bay. Captain Janeway, however, was unsure about allowing him more privileges. (VOY: "Basics, Part I")
This progress came to a halt when Kazon-Nistrim agent Teirna detonated a biological explosive on Voyager. The explosion took place adjacent to Suder's quarters and the Betazoid was believed killed. While the ship was taken over by the Kazon, Suder hid in an access conduit and managed to make it to sickbay where he found The Doctor, who was already planning to retake the ship when Suder arrived. During their sabotage efforts, Suder was forced to kill a Kazon soldier. Despite The Doctor's assurances that he had done the right thing, he succumbed to a panic attack as he had been working so hard to avoid his violent impulses.
Suder was instrumental in helping the crew retake Voyager from Seska and the Kazon. After The Doctor was deactivated by Seska, Suder was left alone to disable the backup phaser couplings, but was slightly comforted by a message The Doctor had left him, in which he assured Suder that he had The Doctor's complete confidence; Suder had already shown great loyalty and dedication to his crew. With this in mind, Suder was able to sneak into engineering and launch a surprise attack, killing eleven Kazon soldiers single-handedly. After killing the Kazon that were holding engineering, he was able to sabotage the backup phaser coil, causing it to explode when it was activated (Tom Paris having disabled the primary couplings). He was killed by a Kazon soldier in the process.
Thanks to Suder's bravery, Voyager could be retaken, making it possible for the entire crew to continue on their journey back home to the Alpha Quadrant. After the crew were back on Voyager, Tuvok spoke briefly over Suder's body after The Doctor and Paris had told him of Suder's heroism, offering him a Vulcan prayer wishing that, in death, Suder would find the peace that he could never achieve in life. (VOY: "Basics, Part II")
He would later be named on a list of telepathic casualties Devore Commander Kashyk confronted Captain Janeway about in 2375. He was the only member of the list actually dead, the rest hiding in transporter suspension. Janeway explained to Kashyk that Suder died in a "conflict with a species called the Kazon." (VOY: "Counterpoint")
Lon Suder was played by Brad Dourif.
When asked about the manner of Tuvok's final tribute to Suder, Dourif recalled, "I think that was dead on. It was a short little thing. There was another death there, which I think had more meaning to other people, but it was just a private little thing for Tuvok, giving Suder some closure. And I thought that was also appropriate because Suder was sort of an alone person and only one person recognized him. For a sociopath, empathy is impossible and you almost have to create it intellectually, and you never feel it. But I think people felt for him. Suder tried. He really, really tried." Lon Suder