In the Mass Effect universe, all kinds of soldiers, warriors, and elite agents can be found across the galaxy to get any kind of job done. Drell assassins like Thane Krios are deadly and elegant, while the elite Spectres are the right arm of the Council. Spectres are not trained; they are chosen. And the Council could stand to choose Zaeed Massani next.
Commander Shepard became the first human Spectre in the year 2183, and by 2186, either Kaidan Alenko or Ashel Williams will become the second. They have what it takes, and it is clear that Zaeed does, too. But he would be a rather different kind of Spectre than Shepard.
What The Spectres Can Do
The Spectres were formed just before the Krogan Rebellions, an all-new type of agent dedicated to upholding the Council's will across the galaxy by any means necessary. These are no ordinary soldiers or police officers; in fact, Spectres rarely even operate in groups at all, instead going it alone and using their own methods to get the job done. Spectres have only minimal accountability since they are meant to operate outside the law to complete missions that no ordinary army or investigator could do.
Still, the Spectres are expected to have some restraint and not sabotage the Council or abuse their power, and they are trusted to not go rogue as Saren did. Spectres like the Asari soldier Tela Vasir may even cooperate with other parties, such as the Shadow Broker, to get the right intel for the mission. A Spectre ultimately determines what they do and how they get it done. They're given a mission, which can range from reconnaissance behind enemy lines to infiltrating a terrorist organization, to assassinating a crime lord or tracking down a deadly outlaw, and that Spectre may use any lethal or nonlethal methods deemed suitable.
Overall, Spectres enjoy a lot of operational freedom and discretion in their missions, and while the Council raised their eyebrows at Shepard's rough methods, they still maintained that the Commander could get the job as he or she saw fit. Shepard even met the last surviving Rachni queen and obtained data from it about the Mu Relay, and whether Shepard kills that queen or spares it, the Council will effectively say "very well." All this would appeal greatly to Zaeed Massani, the veteran mercenary.
The Lone Ranger
Spectres can use anything from biotics to advanced hacker modules to firearms to accomplish a mission, and the hardened mercenary Zaeed Massani fits the bill. He's no biotic, but he is among the best gunmen in the entire galaxy, being able to use any pistol, rifle, shotgun or grenade launcher to blast his enemies apart. He also has a lot of experience with bounty hunting and tracking, often taking jobs to track down rogues who duck out of their gambling debts or wanted criminals of all sorts. Zaeed also knows how mercenaries and thugs think and operate as groups, since he co-founded the Blue Suns mercenary army with a certain Vido Santiago, so he knows what he's up against when facing the Blue Suns or the Eclipse mercenary gang. Zaeed has fought for, alongside and against just about everything, and he even knows how to interrogate victims, including Krogan (according to him, attempting to pry off a Krogan's forehead plate is an effective jaw-loosening technique).
Many people, Shepard included, find Zaeed's methods and personality somewhat unsavory. But a winning personality isn't a requirement to become a Spectre. Instead, Zaeed could make a strong case for himself as a candidate, given his vast experience in bounty hunting, weapons use, and much more. It's true that Zaeed sometimes relies on brute force, such as when he blew up an entire refinery on Zorya to flush out Vido and capture him, and he once crashed a Turian frigate on impulse. But for Spectres, that's not terribly unusual, and the Council could appoint Zaeed as the third human Spectre and assign him to the missions that involve the most explosions and gunfire. After all, Spectres vary in their methods and preferred mission types, meaning the Council could send a Salarian hacker to investigate a deadly crime family or send an Asari to assassinate someone, and send Zaeed to blow up a drug trafficking den, for example, or dispatch a gang of Batarian slavers and blow up their ship.
Zaeed may be a blunt instrument, but sometimes, that's what the Council needs, and Zaeed has more than a little experience making up his own rules and using resourceful methods to complete a mission. He doesn't mind going it alone, either; Zaeed can effectively lead a fireteam, but Spectres typically go it alone, and Zaeed can handle that just fine. He doesn't need or want a superior officer from the Systems Alliance or C-Sec to restrain him or give orders. Zaeed his own boss, and that's how the Spectres operate once they're given a mission.