Generally, you should only tell them what their character knows. Some DMs don't even tell them the creature's name until afterwards, they just describe its appearance.
Let them make a skill check to see how much they know (eg arcana, nature, religion, history - depending on the type of creature. Or perception, if there is some visual clue).
Or, if you decide they would have encountered the creature before, or the creature's abilities are common knowledge, then just tell them.
Otherwise, it's an unknown creature, let them discover its abilities by experience. You'll have to tell the players what's happening during combat, so they know if it is resistant or vulnerable to their attacks. For example, "The red dragon hardly seems to notice your Fireball. But it roars in pain as the Ray of Frost hits him."
So here's my issue: I like my character, I like our party, and I don't want to pull a 180 on my character and make him nice or throw away important motivations for him.
Well, it sounds like your character just may be evil, or at least on the evil side of neutral.
That doesn't mean he has to do evil things, especially if he has a reason not to.
And, if he wants to stay with the party, he probably does. You've already had an in-character conflict where one character "stormed out" and another "left the room disgusted." That ought to be a pretty good clue to your character that, if he wants to hang around with these guys, he'd better start to act nice, even if he's only doing it to keep the other party members cooperative.
Even if your character was a complete psychopath who loved kicking puppies, if he was sufficiently smart he should be able to realize that there are situations where it's better to heroically save the puppies instead. And it doesn't sound like your character is anywhere near that bad.
Basically, you're playing a conflicted character. This can be a lot of fun, if that's the kind of thing you like.
It doesn't matter that the cause of the conflict may be (at least initially) external; even so, it's a source of mental conflict for your character. On one hand, your character worships an evil god. Even if his choice is fundamentally based on pragmatic reasons (power!), a pact with Cthulhu will surely have some influence on him, tempting him to more evil acts and means (not that a hunger for power couldn't do that all by itself). On the other hand, he's also fighting to save the world (even if it might be for his own ends), and has teamed up with a bunch of more noble, good and squeamish types to do so. This means that, whatever means he might want to employ in the pursuit of his goals, he now also has to consider their effect on his fellow party members.
And, of course, once he starts to consider the way his actions are perceived by others, he might also gradually come to realize that there's also a side to himself that doesn't like what he's doing. Maybe not instantly, but after a while. Morality has a funny way of growing on you like that — from "I have to be nice or I'll get punished" to "hey, other people are actually nicer to me if I don't act like an asshole" to "you know, I really should be nice just on principle, because it makes the world a better place."
None of this means "pulling a 180" on your character, or at least, not in a way that he wouldn't have good in-character reasons for. Sure, the conflict with the other party members (and possibly subsequent reflection) might be the trigger that makes your character realize that he needs to drastically change his behavior in order to achieve his goals, but his motivations will still be the same as before.
Best Answer
By the Book
I'd start by suggesting she review Chapter 8: Running the Game, Experience Points. Hopefully, the part she'll realize what she's doing wrong after reading this:
The character advancement system is not seeking to model mythical accounts of battle, where the killer gets the glory for finishing off the enemy and nobody can remember the name of any of the other people who made it possible. Killing a creature is not the only way to gain XP. It should be possible to get XP with no killing at all. That clearly makes it unfair to award it for a kill - the non-lethal contributions can be just as important, if not more important, than raw damage. Unless the enemies are being played as unintelligent bags of blood and loot, it's even possible to win an encounter without killing anybody.
A Question of Fairness
If needed, you could highlight unfairness of XP-to-the-Killer by pointing out a scenario like this: What happens with something like a rogue sneak attack critical hit or a paladin crit smite leaving an enemy with single-digit hit points, and somebody else taking it out? The rogue or paladin did the vast majority of the damage. Why should the last hit point be more important than the thirty, fifty, seventy, or more that came before it? Remember, experience tracks ability, not renown.