Hawken, most of you said was a matter of opinion.
The first half was not. Then I prefaced the rest with "As a DM". So? Not being snarky either, but much of what goes on this board is the opinion of people trying to interpret rules.
if a society condones the killing of animals such as cow and sheep for survival, why would it be inherently evil for the vampire to feed on life-stock? Because it's torturing the cow? What if the vampire killed the cow first, would that make it better? I'm not being snarky, I was just wondering.
First, you're presuming that vampires can feed off livestock. There is nothing in the core books that states vampire's feeding habits and rules. Libris Mortis is a supplemental book so whatever is there is optional. Maybe cow blood can't sustain. It's up to that game's DM. But for the sake of your point, let's say they can. Now, the society you mention is a presumably human created society. If they consider animal slaughter as acceptable (not evil), then that is their rules as humans--it is still evil to kill other living beings, but not in the eyes of that society. Why would it be evil for a vamp to feed on cattle? For one, it is intentionally inflicting pain and suffering on another living being. Animals hunt and kill because they are beasts that live by instinct and don't know any better. But for sentient beings to intentionally inflict harm on others, regardless of motive, is a cruel act. Vampires are sentient, they have a choice. They may not have any options, but they have a choice. Their very existence is already unnatural, but for them to continue to exist they have to intentionally inflict pain and suffering on others. They are not born, they are made. The process of their creation could make them evil, but even if someone with enough will was able to keep his "good" or "neutral" moral position, it wouldn't last for long when it realized it must daily inflict pain on other living beings to continue to exist when it no longer
should exist.
You also need to consider that society's rules are for its members. Vampires are not legitimate members of that society, they are outside of it. Vampires prey on people. People are to vampires as what cows are to people. I'm sure if you had a society of vampires, they wouldn't think it was evil to feed on people. That doesn't mean it's not evil though. Just because you can get a lot of people to say something is not evil doesn't mean its not evil, it just means a lot of people are wrong.
And if the vamp just killed the cow first, he probably wouldn't get anything from that blood. Vamps, in no story I've ever read, have ever been able to get sustenance from blood taken from the dead.
I can personally see a person being bitten and turning into a vampire but with strength of will to stay "humane" and limit himself to cattle.
Strength of will is often not enough. A vampire limiting himself to cattle is not something it can control. It's up to the DM of the game to decide if cow blood can actually do for a vamp what human(oid) blood can. And if so, why would the vamp subject himself to that? That would be like Donald Trump eating those greasy hotdogs at a 7-11 every meal when he is surrounded by a much higher caliber of food. When you go to a buffet, you don't eat the mac & cheese, you get the prime rib, crab legs, steak, all the things you normally
want to eat but aren't usually able to. A vampire in a human society is like a hungry person walking around in a buffet. You eat there, you don't leave, go to the grocery store and get a pack of Ramen. Eventually, you'll get sick of it, if you did do that, and that nice, juicy steak (human), just walking around right there is going to seem a lot more appetizing, and heaven forbid that juicy steak (human) bleeds! (Assuming you like steak), when was the last time you had a good filet mignon served up for you, juicy, tender, and with an aroma that just sets your mouth to watering. Did you just push it away and say, "Nope. I'm just going to eat a few gummy bears!"?
And just for the paladin in question, there are no Good religions that condone the consumption of blood, in real life or D&D, afaik. That may not specifically be written in the PHB or Faiths & Pantheons, but I definitely don't see a paladin of any of those Good gods hanging out with a vamp that
says it is not evil. So, for that paladin to hang around with a creature that is a walking sacrilege to his religion and faith is something that could not be possible without the paladin jeopardizing or outright losing his powers.
And if the game is in Ravenloft, then that vamp may think it's LN, but after a few failed Powers Checks from feeding off the living (which would be murder at worst, or at best unprovoked or grievous assault; all of which cause Powers Checks), it would be swiftly going back to the CE it started from.