So I've taken the info and changed the theory again.
Venture basically had an industry/business and was pretty good with it, and his chapters, the Aspirers, followed suit. But then Chaos did its thing, and Venture became more ruthless for profit. Eventually, he would harbor extremely corrupt practices like slavery, extreme pollution, and profiting off of the suffering of others. Some of Venture's men were disgusted at these practices and converted to the Ultramarines, but some stayed true to their Primarch's disgusting cause. Eventually, Emps confronted Venture in combat and slayed him, scattering Venture's marines deep into the Eye of Terror. There they set up a small industrial empire which still manufactures guns, tanks, vehicles, and other heavy equipment for the Chaos Space Marine Legions. They will do any order, as long as it has a suitable price and profit.