While Trunks has become known as a selfless hero who will venture into the unknown if it means saving the people he cares for most, he was originally a murderer who didn’t hesitate to kill someone on sight if they were a known evil-doer–which is exactly what he did when he obliterated Frieza and, by doing so, proved that he’s basically Dragon Ball’s Punisher.
Frank Castle aka the Punisher is a brutal antihero within the world of Marvel Comics who has committed himself to killing every criminal he comes across in increasingly gruesome fashion in the hopes of mitigating the evil of his world. Punisher's dark mission started after his family was gunned down in front of him during a shootout between two rival gangs. After that traumatic event, Frank committed himself fully to being the Punisher. There is no rhyme or reason as to why the Punisher kills people who commit atrocious acts of evil, no personal connection or desires for revenge, he kills them because he is called to–doing so with extreme prejudice and in a way that ensures they will never get back up.
In Dragon Ball Chapter 332 by Akira Toriyama, Frieza and his father, King Cold, came to Earth to enact brutal vengeance upon Goku, the Saiyan who nearly killed Frieza during their battle on Planet Namek. While Frieza survived his fight with Goku, he was badly injured and needed technological enhancements to stay alive–taking on the form of Mecha Frieza. While the other Z Fighters flew to where Frieza and his father landed, another fighter met him on the proverbial battlefield first: Future Trunks. When Frieza mockingly says that he’ll happily kill this mysterious, sword-wielding warrior from the future before he finds Goku, Trunks responds by powering up to Super Saiyan, darting behind Frieza faster than the villain could see, then slicing Frieza in half with his sword. While Frieza was very much dead after that first attack, Trunks didn’t take any chances given the fact that Frieza previously survived a seemingly fatal blow from Goku. So, Trunks slices Frieza up into little pieces then obliterates the villain’s remains with a powerful ki blast, leaving nothing left of Frieza but ash and dust.
When Trunks viciously killed Frieza, he did so because Frieza was an evil tyrant who simply needed to go. Trunks didn’t have any personal connection with Frieza and had never met him before brutally murdering him. In this regard, Trunks is exactly like the Punisher who would have done the exact same thing to Frieza if he had the chance given Frieza’s standing as a universal dictator who was responsible for multiple counts of mass genocide on a cosmic level.
While it could be argued that Trunks only killed Frieza because the villain presented himself as an obstacle in Trunks’ mission to stop the Androids rather than just because Frieza is evil, the manner in which he killed Frieza was far too brutal to not be reminiscent of the Punisher’s methods. When the Punisher means to kill someone, he over-kills them as villains in the world of Marvel have a nasty habit of coming back to life. The same is true for Frieza, and rather than simply avoiding him or sending him away until his job in that time period was finished, Trunks obliterated him and, in doing so, proved that he’s Dragon Ball’s Punisher.