I'm sure this has all been said before but...
Whether your firearm is conditioned for dispatch is irrelevant.
Whether you have an open or closed cert is irrelevant (you will almost certainly not have permission to shoot the specific land, so you could technically be held in breach of that condition)
The only defence you have is one under "preventing un-necessary suffering" for an animal that is injured other than by your unlawful action.
The SNH Best Practice guidance yields the following.
S
ection 25 of the Deer (Scotland) Act 1996 exempts individuals from being guilty of any offences involving the taking or killing of deer at any time if it is done for the purpose of preventing suffering by:
- an injured or diseased deer; or
- by any deer calf, fawn or kid deprived, or about to be deprived, of its mother, or
- a deer which is starving and which has no reasonable chance of recovering.
However that doesn't say that you cannot be guilty of a firearms offence, such as a breach of an FAC condition. I don't believe there is any specific defence for that, but I think that you might successfully argue "reasonable excuse" if your act was purely in the interest of animal welfare in preventing suffering.
Not sure I'd want to be the test case, that's for sure. I have dispatched deer in the past with a knife (no firearms handy at the time) but that was on a country road, late at night and no-one around to see it. If it were the middle of the day, on a busy road, and the police were present I'd be having a serious discussion with them before acting.