AI doesn't get in it's calculate to attack/block decisions that Fetid Imp gains deathtouch if the ability is activated. So will happily attack with a big creature into Fetid Imp with open to activate deathtouch while it will not do the same if Fetid Imp has already deathtouch. Or the opposite AI will not attack with Fetid Imp with open if opponent has bigger creature but will attack if Fetid Imp has already deathtouch.