Why murder, robbery et al. is wrong?
Well, the question of what things are morally wrong falls under the area of normative ethics. There are many theories on that.
There are some general categories of theories:
- Consequentialists think that an act is wrong if it has bad consequences. A subset of consequentialism is utilitarianism. Utilitarianists believe we ought to do acts which most promote happiness and lessen suffering. (How exactly this look depends on the specific theory.)
- Deontologists think that there are some specific types of acts that we ought not do no matter the consequence. They'd f.e. hold that we ought not commit murder, no matter the circumstance.
- Virtue ethicists think that we should look at character instead of acts. So there are various ways in which someone might be bad.
There are many more. But those are the first ones you might hear first in a course on normative ethics.
Is suffering to others a criteria
to decide whether something is wrong or not? - any act which can cause
psychological, emotional or physical pain is wrong.
So, the other questions and objections from you only really apply to theories from category 1 (and just a subtype at that). Utilitarianists think that suffering and happiness are the only criteria for deciding on whether an act is good or bad.
They might employ various restrictions. A Rule Utilitarianist would for example say that we should look at certain types of acts. For example, stealing as a type of act could be argued to be bad because it generally doesn't provide enough benefit (happiness) for the cost (suffering) it has.
But other Utilitarianists could think that we should always look at specific acts. They might say that stealing isn't inherently bad. Instead stealing could usually be bad, but in some circumstances - when it promotes more happiness then it causes harm - actually morally good and the thing we ought to do. For example, maybe we ought to steal expensive medication in order to save a life if we can't afford it legally.
But then what about other creatures or animals in our surroundings?
This definition illegitimate the act of partaking non vegetarian food
on the surface
Yes, there are a number of Utilitarianists that argue that we generally shouldn't eat meat. Peter Singer would be one. (Also, note only Utilitarianists come to such a conclusion.)
but it seems to illegitimate the very act of existence because one
can't exist without causing sufferings to any other creature either
knowingly or unknowingly.
There are a number of ways in which Utilitarianists can argue against this. Here are a few:
- Only normative acts can be wrong. That means we must have a choice. If we don't have a choice then it can't be morally wrong. So any acts that we can't avoid aren't morally wrong. For example: when we walk, we'll unknowingly step on ants. We can't avoid that when living normally. So it's not wrong.
- Existing can be morally wrong but not morally blameworthy. There are justifiable limits to our responsibility.
- Living in a way to cause the least harm to other beings or maybe even commiting suicide is itself harmful. This outweighs the harm we otherwise commit. This needs the premise that humans should be given more moral consideration (for example because we're more sapient).
- We could bite the bullet and argue that existence really is morally wrong. Suicide is too. But we ought to not procreate due to the damage humans cause. This is called antinatalism. It's a minority position.
- There are certain limits to moral demandingness.
Also, the psychological, emotional or physical pain is relative term.
An emotionally weak can suffer a lot from a particular speech while
the speech can't effect any other emotionally strong one. So the
criteria on the basis of sufferings seem vague.
Utilitarianists could deny the relativity in the case. Or they even might incorporate it into their theory.
Furthermore, relative or vague criteria can be good enough. So even if we have no "scale" for wellbeing, we might still be able to know if one act is worse than another. Utilitarianism usually must take some sort of value theory as basis. Although you could argue for it in general without specific value theory, if no value theory in the required way is possible then this doesn't work.
So what is the criteria by which something happens to be wrong?
So, above I sketched out a few responses to your objections. Note that your objections only really apply to Utilitarianists. But the three common kinds of normative ethical theory I mentioned above are roughly equally popular. There's a huge number of arguments for and against each (and the ones that I didn't mention).
Yet, that doesn't mean that none is right.
Also, for Applied Ethics we could choose to apply multiple normative ethical theories: they will sometimes come to the same conclusion. So if stealing is wrong according to the theories that are most convincing to us then we ought not steal. (Note that the question of how exactly we ought to act under uncertainty is a problem area of ethics as well!)