All right, so, how do we ice this guy and make it look like an accident? The biggest issue for me is that Marcus is not the kind of guy who knows a lot about this kind of action; he's pretty direct in his approach. I could see him suggesting something like the below, though.
According to
this article, the most common causes of accidental death in modern times are (and I've eliminated some right off the bat):
1. Falling Objects
2. Drowning3. Alcohol Poisoning
4. Roller Coasters5. Diving6. Electrocution
7. Machinery
8. Medical Malpractice9. Poison gas (not weaponized, but fumes from fuel and the like)
10. Firearms (accidental discharge, not intentional obviously

)
11. Suffocation
12. Fires
13. Poisonous Liquids and/or Solids
14. Motor Vehicle Accidents
15. Falls
Of these, I think Falling Objects and Falls are questionable, unless there's a bunch of construction (of skyrakers) in this little town.
Personally, I think motor vehicle accidents, fires and/or suffocation, and various forms of poison are all viable options.
If we can get a hold of a hacker and a mage (hint hint), we can mess with the driver or take control of the car, and crash it into another car, front to front style. Rigging something to catch fire would involve getting inside (either covertly or as a recruit), and finding a good source. Same with electrocution and/or machinery, and to a degree poison.
Of course, one way to take a bunch of these guys out would be to attack the safe house where this lead is hiding, and use non-penetrating weapons to take people down. That way, after we're done we torch the place and make it look like an accidental fire; bullet holes would be a dead giveaway of foul play, though, so we'd have to use non-lethal SnS or gel rounds only, gas grenades full of breathtaker, and/or injection darts with narcojet or similar.
Personally, I think gas grenades with Breathtaker or Ymir are good alternatives, if we can get our hands on it. Breathtaker because it makes it seem like the victims suffocated, and Ymir because it looks like frostbite, both of which are appropriate for the setting. Warp is another option; dose everybody with a powerful hallucinogen, then go in and clean house, make it look like they turned on each other; messy, but doable. Finally, Neuro-Stun is an old standby, being odor and colourless so we could potentially deploy it without too much issue. If we're going for any of these, we should also inquire as to whether Ares just happen to have, or can get a hold of, some Rock Lizard Blood doses (avail 8R, 1500¥ a pop).
Breathtaker is avail 8R, 50¥ a dose
Neuro-Stun is avail 12R, 60¥ a dose
Warp is avail 12F, 550¥ a dose
Ymir is avail 20F, 800¥ a dose
In terms of torching the place, my bet would be on either letting gas flow into the place (if they have a natural gas mains, or oxygen bottles for welding and/or medicinal purposes), or using some form of accelerator like gasoline or home-made molotov cocktails (they're gangers, after all!) set off accidentally (dropped matches or a nearby chemical heater with a fault).
Electrocution and machinery are more dependent on environment, as I see it; it's up to chance if the location the target is hanging out at has any heavy machinery and or high-voltage power grids.
So, thoughts?