Yes this is a soul-crushing documentary and not technically eco-horror, but everyone I know who's seen Blackfish instantly comments on how horrifying it is...
The basic outline: Current and ex-employees of Sea World tell stories of how they had truths hidden from them regarding their on the job safety, as well the lies they were told about the deaths of their peers. They recount for us how each of them discovered they were being deceived, how intentional it was, and how they feel about it all in retrospect. They also talk to some current employees who are still drinking the koolaide (so to speak) illustrating starkly well how hard employees are pushed to swallow the corporate line.
Cinematic style: Sea World shows are regularly filmed by both spectators and performers. This gives us a film with footage not only from the time periods in question, but the actual shows in which some inarguably horrific events took place.
Good lessons in: Taking the word of people in power, and the amazing truths you can uncover with access to crowd sourced footage.
How this ties into eco-horror: From beginning to end, this movie highlights the callous way people treat living things for a profit.
Ways you can help the cause: The filmmakers website is full of great links!
Lighten the mood a bit by: Intermittently asking “A fire, at Sea Parks?”, ala the IT Crowd.