There are many insane, or temporarily insane, characters in the Yandere universe, with various triggers and moments where they show their insane side. Below are examples of some of those moments.
Though it has been shown that Sam has an 'insane side' in which he denies himself into a frenzy. One such example is the time he believed Grian was Taurtis and Taurtis was Jerry. What began as a way to cope with the fact of stabing his best friend, in which he asks Grian to dress as Taurtis and act like him until his best friend recovers, soon continued into him truly believing that he was Taurtis. But while in the gym he fully believed Grian was Taurtis. Grian for a moment thought he was joking but soon he realized that Sam was actually wasn't kidding.
That's when Rowan began to whisper for Grian to run, much to Sam's confusion, and continued to push Sam into the 'Punishment Pit'. After Yuki stabs and 'kills' him, Grian makes a run for it yelling he is not Taurtis. That is when Sam snapped (near the ending of an episode and the beginning of an episode) and continued to claim that 'Taurtis' (Grian) is driving him insane with his lies and craziness. Offering him Toritos, Grian accepted them just for the sake of not being hurt by Sam or possibly killed.
But then when Rowan survived from Yuki stab, Rowan and Okami tried to calm Sam down but it only made matter worse. Which only made him more confused and angrier as Rowan and Okami tried to shoot him with sleep arrows. But it caused him to get out Yuki's Bloody knife and swing it trying to warn them to back off as Sam ran and tried to hide as he kept repeating himself that Taurtis was here. Yuki as you know is a good person & Sam retreats killing her
And he hid in an old prison and kept saying that Taurtis was his best friend and that he will help him. As he blacked out. It may seem as he repeated that Taurtis was here, Taurtis was probably his first friend he met. As said by Sam's dad, Stanley said he didn't have real friends when Sam's was younger and only had imaginary unicorn friends. So Sam was probably known as a lonely kid back then.
The first time we see Yuki snap and show her yandere side is when she is dumped by both of her dates at prom. While she copes with finding that Taurtis has left her for her rival, she reacts much more extremely to hearing that she and Sam are no longer dating.
At the end of the first season, in episodes 78 and 79 we find that she has snapped again and killed Paul Blart and then Rowan. To appease her, she demands that Sam bring her Grian, so she may sate her blood lust
In the first season, Chan is suspicious of Yuki and acts suspiciously herself, eventually breaking into Yuki's house with a knife and confronting her, but being arrested by a SWAT team before she could do any harm. Later Chan is released from jail, however, as there was not enough evidence to convict her, and she ends up becoming good friends with Yuki.
As a war veteran suffering PTSD, Rowan has certain triggers that bring out his aggressive side. In episode 39 of season 1, we find that his triggers include the smart suit that Sam was wearing for photo day, the song "My Little Buttercup" by Steve Martin & Martin Short, people reading, and camera flashes. Episode 7 of the prequel season, Yandere, has him reacting to a mention of "war", causing him to attack a disabled man who is lying on the floor.