For assisted suicide, I think you just need to make sure it’s the only option left to stop or prevent the suffering of a person (like an incurable disease, or debilitating conditions with no cure, etc.). You also need to make sure the choice is made with enlightened consent.

To allow someone to kill someone else is another level of complexity. The processes of gathering consent, and the reasons to proceed are extremely complex to make sure the decision is taken within the bounds of actual consent, especially if the person to be killed is not conscious or in a capacity to understand.