Firstly, I just want to talk about how much I love the extremes they are going for with the "Metal" aesthetic by having these one-shots have 1980s metal band names like "The Red Death" and "Murder Machine." I really dig it.
There are worlds in the multiverse that are not meant to exist. They exist under other worlds, and things occur in them that are not meant to be. When these things happen, the worlds collapse.
In one of these worlds, Batman loses his family, and he wants to fix things by taking the Speed Force powers away from the Flash. Barry knows this is a bad idea and sees the world falling apart around them. However, Bruce is too far gone, and he seizes the super-speed away from the Flash, the two fusing and becoming the Red Death.
These one-shots are origin stories for the cenobite-esque Batmen that appeared at the end of Metal #2. This is pretty reminiscent of the Justice League one-shots that accompanied Darkseid War. As such, I imagine they won't have that much bearing on the greater story but will be interesting vignettes nonetheless.
As such, The Red Death is a fun little read. It shows a desperate and beyond-the-edge Batman, which is the way I kind of prefer him. It also deals with his aging and Gotham devolving into a near-wasteland with shade-wearing anarchists, which makes it oddly reminiscent of The Dark Knight Returns.
Seeing Batman go this mad is actually pretty fun. The unholy combination of he and the Flash is a brutal monster with two psyches fighting for dominance. Unfortunately for Barry and the villains of Gotham, Bruce is the dominating mind.
We see how he came to our world, too, with the help of Barbatos, of course. We also get to see what he did to the Batman rogues on his own world. Expect a lot of evisceration.
Carmine di Giandomenico's artwork, with his experience drawing Flash, brings a kinetic and grim energy to events that allow for it to live up to Joshua Williamson's sadistic script here. It's a great look, with a good atmosphere aided by Ivan Plascencia's color work.
Those looking for new insights into Metal will only really get anything from the lead-in explanation I recited above. Beyond that, this is just a fun little diversion about Bat-Flash from Metal #2. I can still recommend it though. It's a cruel yet fun read, and you should give it a look — if that sounds appealing to you.