Pietro only appeared once in the episode Impact. It was to prank Lance using the stone Mystique statue. This is going to be what happened before and after Magneto's "death".
Where Pietro Was In Impact
Pietro was in his room, laughing about the prank he just pulled on Lance.
"Haha! That was priceless! I wish he could've seen the look on his face! Ha!"
He, Todd, and Fred all laughed when the Mystique statue fall on Lance. Even Wanda smiled a little.
Mystique was stone now. Thanks to Apocalypse.
Pietro stopped laughing and sat up.
'He's so powerful. How can someone like him be stopped? Is it possible to stop him?'
He thought of his father, Magneto, who was at the Sphinx right now with his Acolytes, searching for a way to defeat Apocalypse.
Pietro narrowed his eyes.
'He put me on his team then the next second, he kicks me out! Saying he needs to lead the Brotherhood. That lair! I know he just wanted to get rid of me! How could he! After everything I did for him! And he doesn't want me to help find a way to beat Apocalypse! The two of them are the same! They're both bullies and madmen! I hope they kill each other in battle!'
CRASH!
Pietro ran down stairs and saw Todd and Fred fighting over Mystique.
"I have her until three o'clock!" shouted Todd.
"She's going on the roof!" yelled Fred.
"Will you two shut up!" screamed Lance.
"Make us!" exclaimed Todd.
The house started to shake.
"I'm going for a run," muttered Pietro, getting as far from the house as possible.
Pietro stood outside of a store, in front of some TVs. He was watching the fight between Magneto and Apocalypse.
Magneto used his powers to send a satellite at Apocalypse, but the powerful mutant sent it right back. Magneto was able to dodge it and now looked at Apocalypse in shock.
Apocalypse held out his hand towards him and Magneto was drawn towards him no matter how much he struggled against it. Apocalypse closed his hand into a fist and Magneto screamed in agony. A sphere appeared around him and exploded in a flash of light. When everything cleared. . . he was gone.
Magneto. . . was dead.
'He's dead,' thought Pietro. 'He's dead.'
He just stood there, letting it all sink in.
Then he started to chuckle. His laugher got louder until he was just roaring with laugher, tears of joy streaming down his pale cheeks.
He wiped his tears and sighed, happy.
'He's gone! He's finally gone! I'm finally free!'
He rushed home.
"Guys!" Pietro boomed once he entered the living room.
"Oh, Tro, something horrible just happened!" exclaimed Todd.
"What? What's wrong?"
"Mystique is gone!"
Pietro's shoulders moved up and down as he silently laughed.
"I never thought the day would come where anyone would be upset that the blue bitch was gone."
"I'm only upset 'cause I wasn't done gettin' payback, yo! I was gettin' back at her for all the times she treated me like dirt!"
"What happened to her?"
"Dunno, yo. She was just gone."
"Well, I have some fantastic news to tell you and the others. Let's bring them here."
Once everyone was in the living room, he told them what happened to magneto.
"Seriously?" asked Lance. "He's dead?"
"Dead," replied Pietro, smiling.
Wanda burst into tears and ran upstairs.
"Oh right. Forgot Magneto brainwashed her into thinking he was an awesome father. Oh well. I'll help her realize he was the worst father ever later. Right now, let's celebrate!"
"Yeah!" agreed Todd.
"No more buckethead!" cheered Fred.
Lance was quiet.
"What's up, Lance?" asked Pietro, taking a seat next to his best friend. "I didn't think you'd be upset by this."
"I'm not. I'm glad Magneto's gone," said Lance. "It's just, it Apocalypse could take out Magneto, and your dad was really powerful, can Apocalypse be stopped?"
Pietro thought about it. It worried him too.
He waved his hand. "Let's not worry about it right now. There's no more Magneto and no more Mystique to busy us around."
"You're right. Let's party!"
Pietro ran to the kitchen and pulled out some beer. He opened one bottle.
'To Magneto's death.' he thought. 'May he rot in Hell.'
Pietro took a sip.