You have to resolve the "draw 2 cards" first, by then it's already too late for Peten's effect to activate, since the last thing to happen is no longer Peten being sent to the Graveyard, but you drawing 2 cards.
card's not even working right. Got destroyed by "Aromage Rosemary" didn't even receive input to active the effect. Also "timing is missed" when tributing both 'legion fiend jester' and peten to summon a monster. What rubbish.
Peten is basicly bubonic vermon but speellcaster and slughtly different activation. Rosemary counters effect monster's so i dont know wtf u expect. Use it as it was intended or move the 🔥 on with your life. This is a great card. Well if u know what its for that is