It's time for another round with the guys in the masks.
A radio announcer who tries to poison all of Inland City with chlorine gas for unclear reasons. Ends up blowing himself up while tangling with heroic pilot Captain Steve Ransom. (Keen Detective Funnies v2 006, 1939)
This unnamed fellow takes the cake for audaciousness: his racket is drug smuggling and though traditionally one wants to keep a low profile while attempting to move controlled substances from place to place he:
-sets up camp in the basement of an occupied house,
-keeps a gorilla in that same basement,
-sends that gorilla out to attack randos in hopes of driving away the locals,
-poses as an FBI agent in an attempt to throw Power-Man off his scent.
All of these are what I would describe as "activities likely to invite scrutiny". And they do! (Fight Comics 005, 1940)
Muipo here is a Japanese agent in all but name working to foment trouble in China by flooding the country with opioids. His major interesting features are a decent looking half-face draped cloth mask (rare and annoying-looking to wear) and a fake fat guy body he sits in while dealing with his underlings. He gets beat up by US Marine Strut Warren precisely one page after making his debut. (Fight Comics 007, 1940)
The Squadron of Death is a group who blow up US defense infrastructure for... some reason. And there's the problem: very little is given away about who the Squadron are or what their motivations might be (beyond general anti-US sentiment) in this issue, and though there is some indication that they might return to vex the Arrow in future instalments the next three issues of Funny Pages are not available to anyone without about 10 to 15 grand to spend on physical copies.
Pending further information the Squadron of Death are just two guys who steal a plane and then variously get thrown out of it or blown up in it by the Arrow. (Funny Pages v4 004, 1940)