Spoilers, no spoilers?

An Air Canada DC-8 crashed in 1970 because spoilers were deployed
before landing, which "by the book" is a definite no-no (cf. Air
Canada 621 disaster). How come the first officer (with over 5500 hrs
with that type of aircraft) made such a decision to deploy spoilers
while still in the air? What else, when full flaps and idle power
aren't enough, can a pilot use to handle a too high/too fast final
approach if not deploy spoilers?

As a side question, what is the functional difference between spoilers
and airbrakes?