That is a difficult one, and I don't think anyone can tell you for certain whether this experience means vet med is right for you or not. I do think being exposed to euthanasia etc for the first time will always be difficult, and your reaction to this doesn't mean whether you're right for vet med or not. I near fainted watching a spay for the first time at 17 and it certainly didn't mean I can't handle surgery generally. With euthanasias generally, most vets you'll ask will say they kind of get desensitised to it - it's obviously still sad especially for the owners and even moreso if it was an animal they knew well, but the actual act of it is quite a common thing carried out in practices obviously. It is easier if it was 'the right time' in terms of the animal was very sick etc as you are ending their suffering, which I would say is the more common reason for it happening - if you are ok with those situations (+ accepting those would get easier over time more than likely), I'd say I wouldn't rule out vet med.
As for behavioural euthanasias as in your case, I do understand that is a harder pill to swallow - I think as a vet you can choose not to do those (I don't think I would for certain cases), although probably best if there is someone else in the practice who would. But you can also think of it as preventing suffering, as with quality of life euthanasias - had the dog not been euthanised, would it be passed around from pillar to post, being stressed in new ownerships all the time? Would it spend its life in a kennel with minimal stimulation etc? Would it be always be treated well? Would it be abandoned? The behavioural issues could also be a result of physical health issues which may not have been diagnosed or necessarily treatable (brain tumours etc). Everyone's take will be different on this of course, but imo death is not a 'negative state' to be in, whereas there are certainly negative states the dog could end up in if it is not euthanised.
If you do decide that vet med isn't right for you, I wouldn't think of the experience as wasted - if it showed you that you indeed don't want to go into vet med, then as you said those 2 weeks will have prevented you from wasting 5 years at uni and potentially more years post graduation. If your next placements aren't at vets, perhaps they're still careers you could be interested in? If not I'm sure they'll understand you cancelling. I do understand the frustration on the chemistry part - I also despised it and only took it for vet med, but perhaps that still opens up some avenues to other careers which may require that A Level?
But I do think this is something you need to figure out yourself, and there is no right or wrong answer. Maybe speak to your friends, family, teachers etc for more insights as they'll know you better personally. Could you reach out to the vets you were with (or another vet) and maybe have a little chat with them about it? I'm sure they'd understand you were feeling a bit down after what you saw and they wouldn't want to leave you feeling this way, so maybe they'd have advice.