Not commenting on the overall viability of shooting a rocket by competition (it would be stupid IMO), but it's not just a single launch ruined. Launch failure can stop launches/production for months (3-4 months in this case, 5 in the case of the previous SpaceX failure), or even longer if finding the cause of the failure is very hard. A pause in business like that can destroy a company that needs cash. Also, it reduces customers' confidence in the failing rocket, which would be good for the competitor with a flawless record.
Compare that with Boeing's MCAS problems, the plane crashes themselves were less financially damaging than the resulting groundings, production pauses and canceled orders.
It could have been a single person acting alone for whatever reason. Plausibly an employee of ULA who formerly worked at SpaceX and was mad as mistreatment there, or mad they interviewed and didn't get hired, or mad that SpaceX was beating them at whatever and making them look bad or whatever.
Or it could have been a person unaffiliated with ULA who just found a way to access their roof.
History has a handful of pretty significant events that were basically "one guy with a rifle".
It's completely kooky, so kooky in fact, there's a (IMHO very funny) sketch about turning business competition into a gunfight: https://www.youtube.com/watch?v=mpC_hO15IoA. It's funny because the idea is so absurd.
United Launch Alliance has lawyers. Blowing up your competitor's rocket with a gun would be about the stupidest thing you could possibly do. The upside is low (single rocket launch ruined), the downside is massive (huge civil and criminal penalties). It also suffers from the typical conspiracy theory weakness: everyone involved would need to keep their mouths shut forever, and realistically, at some point someone's gonna have loose lips.