IN MY OPINION, ABOUT YURI PLISETSKY...
Yuri Plisetsky is the definition of teenage rebel. He spends practice time on his phone, he’s a little brat to everyone except for the few people he likes, and he acts grumpy like it’s in style. Throughout the series, he’s had this cool exterior and has been dead set on not letting anyone in and even trying to end relationships before they begin (see: Yuuri Katsuki). He would do anything to protect his image of being the best, and this is clearly shown in the WTTM manga where he says he was upset Katsuki beat him in the free skate scores even if he ended up winning gold.
Yuri Plisetsky wants to be the best and feels he has a lot to prove.
When he saw Victor go on the ice during Yuuri’s exhibition and Yuri went wild, it was no surprise he would end up going wild and doing something to completely clash the feeling of Victuuri’s skate. Yuri not only wants to be the best, but he wants to be remembered.
Which is why Welcome to the Madness makes sense.
He seems to be showing off the signs of teen wanting to grow up too fast syndrome, which everyone goes through. This could be his breakthrough moment, showing that he can be the Eros he once desired in the beginning of the series. He choreographed it himself for crying out loud.
His creation of WTTM could come from the fact that he is inspired by Victor Nikiforov, shown in how he wants him so desperately to be his coach in the beginning. Victor was always striving for a new angle, to surprise the audience, and Yuri could have picked up on that. He could have wanted something completely different than the purity of Agape and the beauty of Allegro and decided to do a 180. He wanted to show how good of a skater he was by skating to different types of music and expressing different emotions. He wanted the audience to feel something as he skated.
I don’t feel that WTTM is out of character for him, in fact I was the opposite of surprised when he came onto the ice with his dark clothes and sex appeal. He’s just a boy who wants to prove himself and be as well versed as he can and I think the creators did an amazing job showing the dynamic of his character with the addition of his exhibition skate.