The Robo-Emperor's design looks less like a medabot than a feral, mechanical, fully-armed Lamia. Perhaps, it was even clearly inspired by an EVA.
In Episode 51, Metabee experiences a dream state of a familiar but very distant past. He battles in a seemingly endless war against Rokusho and an army of Zorins, which also shows him the many casualties of this long-distant war, including the death of a dear friend of his (who happens to be his only known friend): a Sailor-Multi, who resembles the present Brass in voice and model.
The Ankle Biters and their Churlybear. They appear innocent at first, luring a potential opponent into their "game", challenging them into a robattle. If the medafighter doesn't know how to make a strategy to handle and defeat Churlybear's gravity-based power attacks, they are in very deep trouble.
Dr. Meta-Evil has successfully gotten control over all the Medabots with beetle medals at the Miyama Ruins thanks to Metabee's rare medal. The next scene featuring him after Operation Beetle Mania starts, the room seems more empty save for Dr. Meta-Evil and his cat. Had he already announced the Medabots to kill the other Robos in the room?
It also doesn't help that after Ikki, Samantha and Koji arrive with Peppercat, Sumilidon and Rokusho that we see what is probably a lifeless Robo on the floor between our six heroes and Meta-Evil, confirming that Meta-Evil's command to the Medabots also meant no one in the Rubber Robo Gang was spared. Makes you wonder why anyone would want to continue being in that gang after this event takes place.
The "Ten Days of Darkness". An event that happened during the last few days of the World Robattle Cup from eight years ago, where everything proceeded normally, until a medabot invoked the Medaforce. This medabot was unknowingly being used as an amplifier to control other medabots not only in Japan, but around the world. They were forced to rebel against their medafighters (and humans, in general) and who knows how many were victims or how many actually died in the process. Although it was once stopped in the past, this event was later attempted to be repeated twice.
It's implied that Victor lived in a small village with his family and Warbandit, in Kenya. His village barely survived during this event and he is also implied to be one of its few human survivors. Sliding into Fridge Horror, during Victor's flashback, the viewer sees not only a few other Ax-Crazy medabots like Warbandit, but also others that could have been used as "war machines" or weapons by an army, before the "Ten Days of Darkness" even stroke.
Spirits has an episode featuring a medabots junkyard full of zombies with a cute female medabot as their leader, who is very damaged. Her eyes even look like they are "tearing" blood that is actually oil.
The scene where Totalizer gets attacked and brainwashed plays like something out of a horror movie with dozens of hands shooting out of the ground grabbing him.
The Kilobots have shades of this. Imagine all the bad things a rogue Medabot can do (read above on the "Ten Days Of Darkness"), and then imagine said rogue 'bot being built by and for "Stop Having Fun" Guys that are so obsessed with winning that they equip their 'bots with Weapons Of Mass Destruction. One episode involves a runaway Kilobot that is so powerful by its lonesome that the police and the military are completely useless in their attempts to stop it (and not for lack of trying, either), and demolishes a good chunk of the city in its rampage.
Other media
The manga adaptation of the videogames (at least the first four serial games and Medarot 7) has always been published in magazines which demography is mainly children, but that doesn't mean that there aren't going to be visuals that may invoke horror, especially if the reader becomes attached to some of the characters.
The Medaforce is invoked through a very, verypainful process for a Medabot, hinting that they aren't normal robots: it boils their body armor to the point of near destruction, while their minds are scrambled to the point where they attempt to fulfil their new urge to destroy anything.
When a Medabot gets shot or gets sliced by their opponents, oil is splattered as if it was blood.
During the World Robattle Cup, Hikaru's Metabee in Medarot 1 had a particularly ugly fate right after the Rubberobo takes over the tournament. It's literally blasted in half from the waist upward, complete with fried internal framework and circuitry sticking out of where the shot impacted, and it still frozen in place like a statue.
The Game Over screen in the GBA games, which occurs when you lose a story-critical Robattle; such as against the Screws or the Rubberobos. Ikki falls from the top of the screen with a horrified expression and a Scare Chord, hitting the ground and collapsing unconscious against a black-and-red background as depressing music plays. It's a pretty effective Jump Scare when you aren't expecting it; especially when you're used to the whole "just losing a part" scenarios that previously losing had, or "whiting out" as you would in Pokémon.
for a Medabot, hinting that they aren't normal robots: it boils