[you are a nurse who is part of the Avengers]

Whilst you were waiting on the Quinjet for the others to come back, you hear gunshots from their direction.

You & your crush bolt to where they came from, seeing Clint & Taylor both on the ground with a bullet wound, and Pietro holding the gun. 

[eventually you find out he had been under Loki's control, after Pietro had been in a cell for a few weeks]