Source: Futures End #0 (2014)
Type: Alternate future
First, if you'd like to read a good blog post on why Futures End is terrible, there are plenty out there, which is why I didn't feel the need to add too much to the pile. Check out Anj's post at Supergirl Comic Box Commentary. It pretty much covers all the important bases.
For me, the New52 hits a new high in homogeneity with this stuff. Evil psycho versions of DC characters killing each other violently is something you can already read regularly in Injustice, or Earth 2, or Justice League 3000. A dark dystopian future that must be prevented is what this version of the DCU sprang from LESS THAN THREE YEARS AGO. Since then, in addition to the dystopian futures prevailing in canon and non-canon comics, the "save the future" trope has JUST RECENTLY been used by Marvel in their Age of Ultron event, which also featured killer robots. At this point, saying it's Terminator is disingenuous. Futures End is much more cynically derivative than that. And guess what? Because Batman Beyond goes back in time too late, to yet another dystopian future (or do you believe for one second Green Arrow will be killed in 2019?), he'll end up going back further and undoing THAT timeline as well. What's the point?
And it's the absolutely wrong thing for DC Comics to put out at this point. Not because it cements its reputation as the gloomy, ultraviolent, po-faced alternative to DIVERSITY IN COMICS, but because the New52 universe is still in its infancy. We don't even know what makes half these heroes tick and yet we're intent on deconstructing them as if Watchmen and The Dark Knight Returns had just come out and been surprise hits. A dark alternate version of Superman (or Wonder Woman or Firestorm or whoever) doesn't feel all that shocking when for old-time fans, the BASELINE versions of those characters still feel like ALTERNATE versions. DC has to let them settle into their new paradigms, and that's just not happening when crossover events dictate they should face competition from presumably "cooler" Elseworld types, or be pushed out of their own books by villains.