I am not necessarily Buddhist but I am a vegan and killing doesn't sit well with me. I have suffered terrible sleep condition over the past 2 years and can't work in my field anymore and have no experience in any other field and have a hard time getting to work at an early time so midday work is best for me. Unfortunately the restaurant I work at currently has oysters for sale and they are alive when you prepare them which involves killing them. I don't know how to rationalize this but I am struggling financially and am working on getting a different restaurant job where they might not serve this dish. But I have killed some oysters already. How do I come to terms with this?
The Buddhist teachings do not exist for you to create suffering for yourself. Oysters are extremely primitive life forms. While they breathe oxygen, they are very close to plants. Oysters have a nervous system but not a brain. We practise non-killing as Buddhists so we cultivate a respect for life and cease to be violent people. If killing oysters (that don't suffer or don't have relatives or loved one's who suffer about them) is against your moral principles then find a new job. However, if you think deeply about what or who you have harmed, I don't think much will be found. Don't worry about it. Let it go. An oyster does not have a brother, sister, mother or father somewhere in the universe grieving about their lost relative oyster. The Buddha appeared to infer in SN 12.63 that lifeforms consuming other lifeforms is part of the cycle of life. As a "vegan", I imagine your views about food are more extreme than the average Buddhist. For example, the Buddha did not preach vegetarianism. The above said, personally, I would not like to kill oysters for a living.
According to the Bhikkhu Patimokkha (quoted below), which are rules for monks, killing a human is grounds for immediate and irreversible dismissal from the monastic order (parajika).
However, deliberately killing an animal is an offense that requires only confession (pacittiya) within the monastic order, usually with the intention not to do it again. It's not as severe as killing a human.
If possible, change jobs or avoid doing the task of killing oysters. This is because killing oysters involves generating the unwholesome deliberate intention of taking a life. It's against the five precepts and Right Livelihood.
Also, don't carry remorse in your mind, which is unwholesome and unskillful. Confess it i.e. acknowledge that it's wrong then let it go. Don't hold on to it.
Intentionally bringing about the death of a human being, even if it is still a fetus — whether by killing the person, arranging for an assassin to kill the person, inciting the person to die, or describing the advantages of death — is a pārājika offense. (Pr 3)
Pouring water that one knows to contain living beings — or having it poured — on grass or clay is a pācittiya offense. Pouring anything that would kill the beings into such water — or having it poured — is also a pācittiya offense. (Pc 20)
Deliberately killing an animal — or having it killed — is a pācittiya offense. (Pc 61)
Using water, or getting others to use it, knowing that it contains living beings that will die from that use, is a pācittiya offense. (Pc 62)
Tell it your boss, simply "I observe the precept of not killing, not to encourage others and not to approve. It would be good if my person could take on tasks which do not involve such.", and simply abstain. Mental and future struggles from wrong doing are by far more hurtful and lasting then whatever hardship out of earn.
Not out of causes the Buddha told that aside of killing, involvment in business with meat or living beings, does not fit well for someone after long time happiness. And whatever gain, of cource needs to have it's sacrifies: why not letting go of what hurts?
[Note: this is not given for trade, exchange, stacks, but for release. For discussion and further question one is given to make use of this exit.]
Since you're not necessarily a Buddhist, I'll try to answer from a more vegan point of view although I'm not a vegan. You can only come to terms with this by telling your boss your beliefs and stopping. You can't continue to uphold vegan beliefs and kill animals. I know your financial situation is at stake but you don't want to be a hipocrite. There is a gentlemen I supervise who we excuse from killing bugs which is a function of his job, that we excuse because of his spiritual beliefs.