Why heart beats increase when we are in a state of tension even though it is a disadvantage?
The heart rate increases due to any acute stress, it is part of the flight or fight response. This is part of the responses that readies the body for rapid extreme effort. 

The body is not selective about it, any acute stress causes it. One idea why is that a hormone response is slower than a direct nervous response so the preparing the body needs to happen as fast as possible since it takes longer to complete, compared to an actual identification, decision and movement. 

Also remember that only for a small number of snake bites is this counter-productive, for others it does no additional harm but helps get you away from the snake. Snake venoms vary greatly in their effects.