For example, although if I decided I didn't want to live I could not possibly have anything which would be objectively GOOD for me, could other things nonetheless appear to me not to be bad, or to be avoided (such as avoiding a torturer, for example)?
There is a difference between what can appear to be good or bad for us over the short term or out of context, and what really is objectively
good or bad for us in the full context.
Metaphysically, a life is an end-in-itself, because its continued existence depends on one's actions. Biologically, our abilities to feel that something is pleasurable or painful, or good or bad, evolved as capacities we need to survive. But we have free will: We are not required to act for the sake of our lives, nor even do what feels pleasurable. Indeed, we are not even required to act as we judge right. This is why we need ethics: Consistently acting for the sake of our own lives is not automatic nor easy. And because it is not automatic, it depends, at root, on a voluntary choice to live.
What if a person decides he no longer values life? In the instant of that decision, would his ability to feel pleasure and pain disappear? It would not. How could it? Pain and pleasure are bodily mechanisms, built into our nervous system.
What about his emotional reactions? Emotions relate to subconscious commitments for certain values and against other disvalues, commitments that develop over time and that are hard to change. One’s emotional dispositions cannot disappear on a whim or an instant.
Given these facts, could a person who says he rejects life continue to try to seek values based on what feels to be right? Sure, he could. Many people shamble through life in just such a way, at least until their self-contradictions and self-loathing destroy them and their values, or until their emotional mechanism realigns with their conscious nihilism and they end up desiring death.
Bear in mind, though, that truly deciding against life is to decide that no pleasure, no value, and no happiness is worth pursuing. So someone who looked at his life "and didn’t want that" (as you say in your question) would hardly want to seek the pleasures of life or seek to avoid the pains; he wouldn’t want any of that, after all. The flip side is that, since pleasure, value, and happiness are aspects of the experience of living, to aim for any of those is to prize one’s life—if one understands one’s actions in the full biological and metaphysical context. So an attempt to cut one’s values off from a foundation in life, while going on living, may just as well lead one back over time to appreciating life as a whole again.
It’s not very realistic, then, to imagine that a person has intellectually decided for death without already having suffered emotional or physical pain, or already having travelled down a self-destructive path in various aspects. Many actual suicides suffer from depression; their emotional malfunction is the real source of the problem. Rather than suffering an intellectual distaste for life, they feel cut off from the experience of living and valuing. They can’t rely on their emotions for rough-and-ready guidance and motivation, as we normally do.
Relatively few people consciously make all the connections of the Objectivist ethics. But the vast majority do choose to live; they recognize that bad courses of actions are unhealthy, and actions leading towards one’s death are presumed to be wrong. This sense of life as a basic good, even among people who embrace explicitly self-sacrificial ethical codes, helps people keep their value-commitments, social relations, and life-commitments generally headed in a positive direction. There are times when large numbers of people reject the basic value of life for the sake of something else (a nation, a race, or a faith, for example). Those who value life recognize those as horrible times of war, destruction, and suffering. So although there are relatively few Objectivists, and even Aristotelian ethics is not explicitly followed by many, most everyone knows that no one’s good is served by a Sunni suicide bomber who kills a group of devout Shiites in Iraq.
Deciding one doesn’t value life as such doesn’t strike one down in an instant or transform one into a zombie. The effects are longer-term and sometime subtle, but they are real nonetheless. Contradictions do not exist, and embracing a contradiction at the root of one’s morals can only do harm in the end.