I'm not saying this is always the case. I'm saying that unless you're completely desperate or absolutely know otherwise it's the safer assumption.
I nice worked for a Big 5 company. I'm not aware of Machiavellian efforts to destroy competitors; they just bought them.
I've worked for an international software company with ~1,500 staff. I do not think our management were inclined to collaborate with their competitors unneccessarily; they had a fiduciary duty to their shareholders.
I've worked for two small-town website developers, with say 10 staff. There's four or five competitors in this town. My management were happy to collaborate with competitors to stage conferences; but I know in both cases they wanted to crush them.
None of these jobs was in SV, or even in the US.
I note the story about IBM's threats (above); so my limited view is that aggressive behaviour between companies occurs across the range of scale.
Incidentally: it doesn't make you seem weak and defenceless that you seek a collaborator in fighting a patent troll. The trolls are backed by venture capital; you are a sole trader. That you are considering defying them is courageous.
Even taking into account the different business culture of the 1980s, Microsoft's early ruthlessness is well known precisely because it was not the norm then, and certainly not the norm now.
Telling a competitor that you are being sued by a patent troll does not impair confidentiality. And if they are a true competitor, it is very likely that they have also been sued by the patent troll, or are next on the list, and may join in multi-party litigation against the troll.
You signal to your competitors the very weak and vulnerable position you're in.
Maybe in your particular industry or geographic niche its dog-eat-dog.But especially outside of tech, most companies will band together against outsider threats.
I wouldn't put it past some Machiavellian type competitor to tip off the troll to let the troll take you out for them
The only way this would happen without backfiring on the plotter is if they were already targeted by the troll and lost.
Businesses aren't as ruthless as you seem to think they are. They are run by real people, and they and act like people. The kind of ruthlessness you describe is something you see at the largest levels (i.e., Amazon and Apple) where sociopathy is a virtue rather than a hindrance, and even those companies will band together against patent trolls.