1st of all, obviously the other player that was in a hurry to leave, told you that he was in a hurry. Therefore all you did was use the information that he gave you to your advantage and you did it legally. He was not very smart in telling you that he was in a hurry, so that is his fault. The timer is there to be used and you have the right to use it to its full extent or not as you wish. Even if you were not multi tabling, based on the information that this player gave you, you should have done exactly what you did, hoping that he left the table sooner rather than later and then you could take out the low stack and then just take him out easy as a "sitout". Of course if you were to get some premium hands like AA, KK or QQ it would be very tempting to call his All In. ;-)
After reading many of the replies, so many people do not seem to have read the posters original post. He is at the Final Table with only 2 other players! The short stack has 8 BBs and the person who is playing fast all ins has 30 BB to the poster's 40 BB. The poster was 100% correct in using the timer to his maximum benefit. Why give the Fast All Inner who needs to leave soon, a chance to double up and take 1st place???