tl;dr: Relativity + Vegetarianism
... > Breatharian > Waterian > Fruitarian > Vegan/Vegetarian(by choice) >= Vegan/Vegetarian(by birth) > Eggetarian/Lactarian > Lacto-Ovo Vegetarian > Pescetarian > Pollotarian > Non-vegetarian but no beef > Non-vegetarian > Cannibal > ...The case
I am a Non-vegetarian.
I have many Vegetarian friends and most of them do not wear the opinion that they are right and Non-vegetarians are wrong. That said, cases aren't rare where some occasionally radiate from their subconscious the aforementioned assertion that Non-vegetarians are wrong.
The argument: A more righteous Cannibal and a less righteous Breatharian
"I cannot believe it. You people kill animals!" - It would not be totally unjustified if a Vegetarian exclaims this out of a sudden realization. Imagine a Vegetarian who maintains: 'Vegetarians are right and Non-Vegetarians are wrong'.
Is he right about being right?
Now make the following replacements in the above chunk of text:
"animals" to "plants"
"Vegetarian" to "Fruitarian""
"Non-Vegetarian" to "Vegetarian"
Is he right about being right?
Do plants feel pain? It can be argued that they do - https://en.wikipedia.org/wiki/Jagadish_Chandra_Bose#Plant_research
I agree that the pain an animal suffers when it is (tortured and) killed is vastly more when compared to what, as far as we could perceive now, the plants would suffer when ripped apart.
My point is, it is senseless to label somebody right or wrong when it comes to morality/kindness/righteousness. Simply because there there is no absolute right and absolute wrong. Still in a sense, Vegetarians can be argued to be more righteous compared to Non-vegetarians in the same way that Fruitarians can be argued to be more righteous compared to Vegetarians.
Fig: Cannibalism. From Wikipedia
If your eating policy as a cannibal is that you will not eat 'those who dwell near you' or 'those who are your friends' or even 'those who worship cows', then in a sense you are more righteous compared to someone who eats all humans no matter who.
Fig: Sun gazing, from Wikipedia
A Breatharian is somebody who does not consume food. They sustain on sunlight. Even while being a hypothetical Breatharian, you can be less righteous if we interpret our definition of things that we care about to include photons also. This because you are, in a sense, killing the photon. Just that you would have to broaden the meaning of 'killing' to include the transformation that a 'lively' photon goes through when you eat(hit) it.
Of course this post probably means nothing in practical reality that we encounter everyday. This is a part of an attempt to pause and look around at everything through relativity. When one does that, he might realize that if you ignore the scale, 'Vegetarianism is right' is logically similar to 'Cannibalism is right' in their own respective logical universes.
I respect many Vegetarians, mainly those, who are so by choice. They can sure be proud of their position. Just my two cents: Be proud but do not forget relativity.
Remember this? 'Stealing is stealing - doesn't matter whether it is 5 bucks or 500 Billion bucks'
As a Vegetarian, you are, in a sense, entitled to that pride of being more righteous than a Non-vegetarian. Just the same way a 5 rupee thief is entitled to the pride of being more righteous than a 500 billion rupees thief.
I genuinely hope this write-up doesn't cause more torture and killings. If somebody is sensitized to the relative nature of righteousness, I consider the purpose of this post served.
 Sometime back I tried vegetarianism for almost an year. Then I succumbed to the realization of the idea articulated in this post and came/went back to Non-vegetarianism. The other reasons were social pressure, convenience and only lastly, taste. Chances aren't so thin that in future I become a Vegetarian/Fruitarian forever.
 They may have water but they sustain mainly on 'the vital life force'(whatever that means). This is most probably BS - Just google, 'Hira Ratan Manek caught eating'!