1. The prior incidents, the attack on officers in which he destroyed the SUV tail light and an apparent attack on officers a few days later in which he claimed a rib got broken demonstrate that Alex Pretti* was an angry man with a proven record of violence.
2. The disarming happened AFTER Alex Pretti attacked the CBP officers, and the disarming and shooting were separated by hundredths of seconds, not separate actions or an "execution". Had Alex Pretti simply taken phone video and blown his whistle and not attacked the CBP officers he'd be blowing his whistle today.
* I'm still not "afraid" to say his or Renee Good's name, DU onlurkers.