footage shows mario valencia walking along a street in marana , arizona .
alleged robber points item that appears to be rifle into air ; shot rings out .
seconds later , officer michael rapiejko starts speeding toward valencia .
his police car mounts the sidewalk , before crashing into armed suspect .
valencia taken to hospital with serious injuries ; released two days later .
police chief has defended officer 's actions , which saved ` valencia 's life '
suspect had allegedly stolen rifle from walmart and threatened suicide .
comes as us police remain under scrutiny for alleged racially profiling .