I've put a lot of thought into this and figured I'd jot it all down somewhere.
I've been trying for awhile to wrap my brain around Etna and especially her actions in Disgaea 2. The manga kinda helped a lot. I also recently completed Etna mode, which really added even more insight into the character.
So, uh, yes. Etna Mode Spoilers Warning~!
Starting Thoughts...
While Etna can be kinda rather cruel sometimes and thoughtless (although it's mostly against her prinnies lol), if it's someone she actually likes, Etna can be quite nice to them... or tease and poke fun at them mericessily. Sometimes it's hard to tell if she's joking or not.
King Kricheskvoy and Laharl are probably the two most important people in her life.
(Ah ha, this part is kinda incomplete... fix it more later \O_o/)
King Kricheskvoy
The King she admired and respected greatly, although didn't understand all his ideals. It's quite possible she did have a bit of love for the King, but she is a bit clueless of exactly what kindness and love is... though not that she isn't capable of feeling and showing them she just doesn't always recognise what she's doing as being kind. When the King was falling in love with Laharl's mother, she says her heartached because she felt the King was going somewhere she couldn't follow.
Sometime before he faced Baal the King asked her to make a promise for him... that she would protect Laharl if something should happen to him. And then of course, something did. When he returned, his power low, he asked her on his death bed to help Laharl become Overlord, something she was able to accomplish.
In Etna mode her promise to protect Laharl is shown as though it happened before Baal, although her journal entries seemed to also place it at his deathbed. Of course she might have even repeated her promise then. Either way it was made whichever order it was done in.
Her promise to the King, to protect Laharl, she takes very seriously and she wants to not let him down, however as it is she feared she already might have disapointed him when she poisoned the Prince.
The Queen
When the King fell in love with Laharl's mother, she found herself hating the queen. Although she tells Flonne later she didn't, which is rather conflicting. (Or perhaps a localization error) But, as she writes in her journal, her hate has faded, but seems unsure why. She wonders if it's time or something else. She makes no indication what... perhaps it's that Etna herself has changed? Or is it because of what the Queen did that has slowly changed her opinion?
The Prince
When the Prince first came into existance she was torn. He was the King's son... but also the son of the women she hated.
She came to hate and envy the Prince. He was clearly loved greatly by both his parents, which probably contributed to this.
However, despite these feelins, even Etna couldn't say no when the King asked her to protect him. It does seem at some point her feelings towards the Prince did mellow a bit to simply considering him an annoying, spoiled brat.
Then she lost her memories, and poisoned the Prince, something she does regret to some extent, but luckily Laharl is not that easy to kill.
She did seem to expect to be punished by Laharl for her traitorous act, but he didn't. That seemed to amaze her. Perhaps this was the first point when even more so her feelings towards him did start to change. Perhaps it was this that was the first hint she needed that he might have any chance of succeeding the throne.
Since then, in my opinion, Etna and Laharl have come to have almost an underlying sibling relationship. They are actually very much alike in some regards. They do often see eye to eye on things, and when they get along and have fun they really enjoy each other's company. Yet they are also both stubborn and quick to anger. They very easily piss off one another. They also both are often in denial when it comes to caring and feelings. While Etna's denial of these things are half as severe as Laharl, she does do it.
Truthfully though I think Laharl and Flonne are both her two best friends, although whether she would admit to it or not is another matter, although she'd be likely to call Flonne a friend before saying that about Laharl.
Of Being a Majin aka Beauty Queen
In Disgaea 2, Etna comes across at first as being suddenly a lot more, well, "hardcore" so to speak. Telling people she wants to kill Laharl. The manga explains a lot that the game didn't. Etna's intentions for Laharl don't seem to be as over the top as they are made to seem. As ever she wants him to be more like his father. I think she had seen hints of it, or she'd never have stuck around as she does, but the pudding thing pisses her off and does go to show he has a long way to go.
According to the manga, that's what this whole quest to become an Overlord has to do with. She wants to show Laharl what a real Overlord is like and knock him down a peg or two. Although she also does worry that perhaps she's being too harsh to him. She understands a lot about why he is how he is, being one of the few people to know about his mother.
Saying she wants to kill him seems to actually hold little fact to it. It comes back to the almost sibling relationship she has with him, as I know I've stated several times in my life I was going to kill my sisters when they pissed me off. XD
Also how she first acts in the game almost seems to me to maybe in part be an act. Etna is quite the actress, and it seems more to be she's merely putting on a show, trying to come across more tough as a demon lord should be. Of course, it's funny since in a way she's failing at why the King was such a special Overlord herself it seems.
As the game goes on, her plans start falling apart and she even starts to consider giving up this whole thing and going back to Laharl, but she's just as stubborn as he is really, not to mention a girl's gotta have some pride. She decides she's at least not going back till she's regained her levels.
Of course then the extra events surrounding Laharl come into play.
How cannon the D2 audio drama is, I'm not sure, since it seems almost an alternate set of events to the in game ones, but are far more amusing. The most interesting story on the first disc containing the trio is the last story where Rozalin seems to aquire Etna's prinnies, probably because they're sick of Etna over working them.
Trying to figure out how to fix it she ends up going to Laharl and, with a flood of fake tears, convinces him to help her with the Prinnies. Both angry they face off with them and with some force they deal with the prinnies and are all getting along, which Flonne points out. And they're both like, oh crap she's right. Suprisingly it's Laharl speaks up first, offering what I think must be some kind of apology judging from Etna's surprised and amazed muttering of "...D-denka." And she goes on with what sounds like an apology too. Yay... then they start arguing again over something and Flonne's just all "Yay back to normal *A*" but lol anyhoo. It's cute.
Either way by end of the game, the good ending indicates she has gone back to the Netherworld with Hanako in tow no less. lol
Would Etna ever really kill the Prince?
No, I don't think she would. Her promise to the King to protect him is too important to her, and truthfully I don't think she'd want to do it either.
In Etna Mode on the PSP and she does kill him accidently, when she realizes what she did when she regains her memories, she actually cries. She regrets it. Although realizes it's too late now. Her choice was made.
H'okay... I think that's it for now till I'm inspired to add anything else. Like Flonne.. oops.