I'm not sure specifically as there could be many interpretations. It might be a dark and fictional scenario where Scott has a motive to kill Peter within the context of a fan - created story.
Peter could use his agility. Since he's super - agile, he can dodge Thanos' attacks easily. Then, when Thanos is off - guard, Peter can strike. For instance, he can web up Thanos' hands so he can't use the Infinity Gauntlet properly.
Yes, there might be such a fanfiction out there. Fanfiction often explores dark and alternative storylines. However, it can be quite a controversial and dark concept as Tony and Peter have a mentor - mentee relationship in the mainstream Marvel stories.
One of the best Kraven stories is 'Kraven's Last Hunt'. In this story, Kraven hunts Spider - Man in a very intense and psychological battle. It shows his obsession and his complex character.
One consequence could be that Peter Quill Parker becomes a hero. People all over the universe might look up to him. But he may also face some jealousy or envy from other powerful beings.
One possible plot twist could be that Peter uses his intelligence rather than just his powers to defeat Thanos. Maybe he discovers a weakness in Thanos' armor or a flaw in his plan that no one else has noticed. For example, he could find out that Thanos' Infinity Gauntlet has a time - limited recharge period for its powerful snaps, and he uses this knowledge to his advantage at a crucial moment.
One possible plot could be that Peter Parker discovers a new power within himself that gives him the edge to take on Thanos. Maybe it's a power related to the Time Stone that he manages to manipulate in a way that Thanos can't counter. He could use this power to outwit Thanos in a battle and finally deliver the killing blow.
Deadpool himself could be considered the main antagonist in this case, as he is the one who is set on killing Peter Parker. His unpredictable and violent nature makes him a formidable foe.
In the fanfiction, it might be that there's a misunderstanding between Deadpool and Peter Parker. Deadpool could think Peter has done something really bad, like endanger the lives of other mutants, when in fact it's a false perception.
Some writers might do it for shock value. They want to create a very different and extreme take on the character to get a strong reaction from readers.