The season 3 premiere of What We Do In The Shadows revealed the real reason why Guillermo has grown into such a talented vampire killer. His aptitude for slaying the supernatural creatures was surprising at first, considering Guillermo is non-violent, not particularly athletic, and is a loyal familiar who desperately wants to become a vampire. Despite wanting to be turned, Guillermo seems destined to be a slayer, thanks to his ancestry; however, his lineage isn't the main reason he has an edge over other slayers.
Guillermo has been a loyal familiar to Nandor and the other vampires in What We Do In The Shadows for over a decade. When Guillermo discovered he was a descendent of Van Helsing, he quickly learned he had inherited the legendary slayer's skillset. The familiar's fate was even foreshadowed by the meaning of Guillermo's last name: de la Cruz, which translates to "of the cross." Ever since then, Guillermo's aptitude for fighting vampires has increased exponentially. What starts as an accidental burning of The Baron via sunlight escalated to the intentional bloodbath in the WWDITS season 2 finale.
But the reality is that ancestry alone isn't responsible for Guillermo's impressive skills. The familiar is amazing at fighting vampires because he's lived with them for so long – and is therefore no longer intimidated by them (and understands their weaknesses). It's to the point where he is even immune to their hypnosis. What We Do In The Shadows season 3, episode 1 revealed this immunity when Nandor, Nadja, Laszlo, and Colin Robinson risked a dangerous four-way hypnosis on Guillermo in order to prevent him from ever harming them — something Guillermo pretended to be affected by, but in reality, he's grown completely immune to being glamoured.
Guillermo being a good familiar to Nandor and the other vampires has given him intimate knowledge of their kind. Because part of his job is to protect vampires, he is an expert in all their weaknesses – which he can use against them. From ensuring there's no sunlight in the house to tucking Nandor into his coffin each night, Guillermo is privy to all their vulnerabilities. The job requires a level of vampire expertise that is not easy to acquire. The other previous familiars in the house all either died or were abandoned — but Guillermo persists, partially because of his dedication but also because of his perseverance.
Not fearing the vampires anymore might be the main contributing factor in Guillermo's slaying skills. Now that he's familiar with their every idiosyncrasy and flaw, he now regularly rolls his eyes at his supernatural housemates. Without this fear, he is able to charge an attacking vampire instead of just running and hiding from it. Guillermo's fearlessness is likely why he can't be hypnotized by them. It's a well-crafted metaphor; he is no longer able to be "hypnotized" or intimidated by their prestige. The irony is that the more Guillermo hangs out with the What We Do In The Shadows vampire characters, the more his talent for killing them will likely grow.
What We Do In The Shadows airs Thursdays on FX and Next Day FX on Hulu.