Bob, Agent of Hydra was going to be my example for the OP but you just beat me to it. 
The reason being, he specifically said why he joined Hydra.
His wife was nagging him to get a job with health benefits. He later found out that Hydra does not offer health benefits.
This reminds me of a tabletop RPG I ran using the DC Adventures system. (While it was based on DC Comics, their stories share a lot of traits with Marvel.) My campaign involved the player characters going to a small (fictional) city that has no superheroes, and becoming local celebrities due to their involvement. I was tempted to approach this realistically… That a bunch of mysterious strangers in bizarre costumes and secret codenames with massively dangerous powers would be treated with suspicion or outright hostility from both the authorities and public. But I remembered that this is a comic book universe, so they were almost immediately hailed as heroes by the public, and law enforcement instantly trusted them and partnered up with them.
My justification for that was simply that this parallel world had built a culture, going back generations, that basically worships costumed heroes and does not treat them the way a normal person would be treated. If you were a “good guy” you were given the benefit of every doubt, and if you were a “bad guy” then nobody would give you any sympathy. This polarization was essentially brainwashed into the populace. In a society like that, who knows what choices this might lead people into.
Sounds a bit like current events if you squint just right.
I had a nod to it by making Lex Luthor the President of the United States (which is also a thing that happened in the DC Comics).
In a book I read recently, the main character, a doctor, meets an actual telepath, something previously thought impossible in that universe. He immediately says that the uses of such a talent are many and obvious: The telepath could be of great value in assessing symptoms in preverbal children, or others who have lost the ability to talk, or communicating with people with locked-in syndrome, or…
The telepath is surprised and relieved that the main character wasn’t thinking of espionage.
I’m not asking why do criminals/terrorists continue their illegal acts despite getting beaten or arrested. It’s why said criminal organizations and terrorist groups (Hydra, AIM, the Hand and countless more) seem to never suffer for manpower despite the sheer number of members that get injured, arrested or killed in a public fashion as a result of different heroes and villains. What’s the in universe reason as to why people keep joining them? If Wolverine dismembers 100 AIM members in a few hours or the Punisher blows up a Hydra base or Deadpool dismantles a drug cartel overnight (and so on), what motivates someone that hears about these events through the news or the underworld grapevine and decides to join said groups regardless?
To paraphrase Harrison Ford:
“It’s not that kind of story, man.”
Tons of nameless mook henchmen are a comic book trope, plain and simple. Unless a writer decides to hang a lampshade on the trope (see Bob, Agent of Hydra above), it’s just a given, and no one worries about “why do non-powered mooks sign up for such a terrible job?”
If this is what takes you out of immersion in comic books, that should be the least of your worries about the genre.
They all have origin stories. But overall, it’s a mistake to try to understand psychopaths and sociopaths in the context of ordinary motivations. They are incapable of the rewards most of us live for, like love and joy. So they are not predictable within that paradigm.
The Marvel villains also tend to be grandiose, which often means they don’t comprehend the possibility of their own destruction. They think the world is their oyster, to be lethally cracked open and the guts ripped out just because they wanted a pearl.
I think the answer was covered from a quote from Dead Again.
“It works because people think that THIS TIME, it isn’t going to happen!”
Same logic for supposedly intelligent monsters, too.
All those fresh new job openings.
When honest work is hard to come by, mookery is all there is. If the industry is not growing, the only jobs on offer are as replacements for people who have left the job. One way or another.
There’s an old saying in the real world that organized crime may be a decent living, but the retirement plan sucks. IRL folks still sign up for that.
Why do the fantasy comics need some other more sophisticated explanation?
He should have joined The Guild of Calamitous Intent from The Venture Brothers, which does offer very good health insurance for its members. The Guild also offers services such as matching heroes and arch-villains, providing henchmen, and training aimless bullies and ne’er-do-wells for a steady and rewarding career in “costumed aggression.”
The US military provides all servicemembers with a life insurance policy. With no exclusions as to how or why you died. IIRC from my era the initial “unit” of coverage was “free” AKA included in your wages and benefits, and you could buy additional units at a pretty good price. I see now that the free unit is no longer part of the program: Servicemembers’ Group Life Insurance - Wikipedia.
Were I a married mook like Bob, I’d be more interested in whether my Evil Superhero employer offered a death benefit than a medical plan.
In our real world, the Mafia never had much trouble recruiting people even though they likely had shorter potential life spans than accountants. Nor are drug cartels hurting for bodies even though their ends are often violent. Same for members of underground resistance groups and terrorist organizations and outlaw biker gangs. It’s even unfortunately true for non-violent immigrants.
The answer is almost always that the life seems better in some way than the dismal lives they see around them. That might be purely monetary, it might be because violence is attractive and this provides an outlet, it might be because like attracts like and being a member of a criminal gang provides similar satisfaction to being a Doper. Why shouldn’t that be true, maybe even more true, in a world with super-large-scale criminal organizations?
One theory: if not for the superheroes (especially ones who don’t hesitate to use lethal force), crime in the Marvel universe would actually be a lot worse, probably an inevitable result of a chaotic, toxic universe, producing an alarmingly high percentage of people who are completely stupid, crazy, short sighted, or desperate (considering the actions of a lot of NON criminals in comic book universes, this doesn’t seem that farfetched). But it’s taken down to something approaching “normal” (by our standards) by a combination of deterrence (people who’d be motivated to crime by economic circumstance or psychological inclination choosing to remain in lives of quiet desperation for fear of being slaughtered by the Punisher), or just culling (people turning to crime, motivated by economic circumstance or inclination…getting caught and slaughtered by the Punisher).
The problem is the Punisher can only be in one place at a time so, if he is in, say, New York City, the rest of the country and world had nothing to fear. Heck, even most criminals in NYC were probably ok. Just bad luck (for them) if they ran across him. Same with Batman.
Best they can do is topple some guy at the top but there is always the next guy willing to fill that role and a few who were a challenge.
Not only is it not free, it’s not mandatory. Every now and then, a servicemember dies in a high profile incident (and surely also low-profile incidents—we just don’t get to read about them), and next of kin learns coverage was declined. For example:
Trivia Note on the Punisher: at least, as of a couple of years ago, in Canon/Earth-616, IIRC, the Punisher is estimated to have killed over 50,000 people (the sliding timescale, and the occasional use of WMDs probably helps).
By contrast, that’s about five times as many people that Ebola has killed this century, or slightly less than the death toll at Nagasaki. He’s actually killed significantly more people than the Joker. And he doesn’t just hang around one city. That’d be terrifying.
In fact…if you go by the Lawful Good Party Line by many superheroes, considering Frank “just another murderer,” it makes them look a LOT worse and more reckless than Batman sending the Clown Prince to a revolving door asylum.
Didn’t Batman have a no-kill policy? Beat the shit out of someone…sure. But never kill them on purpose.