A cluster of nerve cells have founded their own tiny fight club. These cells, nestled deep in a primitive part of the brain, compel mice to aggressively maul not just intruder males, but also females and blown-up rubber gloves, a study finds. What’s more, the fight cells have an intertwined and thorny relationship with nearby “mate” cells, a discovery that may shed light on human sexual violence.
In further experiments, Anderson and his colleagues genetically engineered these fight neurons so that a pulse of laser light would send the same signal to the brain as a rival male would. Male mice don’t usually assault females or castrated males, but a flash of light compelled normally mild-mannered mice to attack: 11 of 16 of these mice charged females or castrated males within five seconds of the light pulse. And six of eight males with the light-activated neurons attacked a rubber glove.

Next, the scientists tested whether they could induce a male to switch from mating with a female to attacking her. As males got closer to the end of a mating performance, it became much more difficult to elicit an attack with the light, the team found. When the nearby “mate” cells revved up, the fight neurons were unusually quiet. Mating quiets the urge to attack, the team concludes.
Small Part Of Brain Itching For A Fight - Science News

There's a lot going on here. For starters it's interesting that mating and fighting are close to eachother in the brain. I'd always kind of suspected that.

I'd also like to point out that this is rather cruel. They programmed Mr. Mouse to kill, but that wasn't enough for the scientists. No. The first thing they think of is to test whether they can make a mouse switch from normal to psycho killer mode halfway through mating with Mrs. Mouse and then they filmed scifi mouse snuff porn, although it gets harder to trigger a rampage as the mouse gets closer to finishing. They didn't just program a living being to kill; they tried to make it kill the ones it loved. That's just cold.