MANDERS. It is the very mark of the spirit of rebellion to crave for happiness in this life. What right have we human beings to happiness? We have simply to do our duty, Mrs. Alving! And your duty was to hold firmly to the man you had once chosen, and to whom you were bound by the holiest ties. (1.358)
In a nutshell we have Manders's philosophy. The emphasis on duty and self-denial stands in stark contrast to Oswald's later explanation of the joy of life.
MANDERS. But a wife is not appointed to be her husband's judge. It was your duty to bear with humility the cross which a Higher Power had, in its wisdom, laid upon you. (1.360)
While he acknowledges the possibility that Captain Alving wasn't what he seemed, Manders doesn't think it matters. His religion tells him that suffering is not only appropriate, it's an honor.
MANDERS. You have never known how to endure any bond. (1.370)
Manders accuses Mrs. Alving of narcissism and self-interest. That may be part of the story, but the other part is the endurance of the bond to her son, for whom she has sacrificed her happiness.
MRS. ALVING. [At the window.] Oh, that perpetual law and order! I often think that is what does all the mischief in this world of ours. (2.41-42)
Mrs. Alving has an anarchic streak that is encouraged by the return of her son. From the standpoint of her character development , we need to see her chafe against law and order, if we're to believe she is capable of euthanizing her son.
MRS. ALVING. But what about the truth? MANDERS. But what about the ideals? (1.58-59)
One might think that truth is an ideal. but in this play, ideals are illusion. Ibsen was a realist.
MRS. ALVING. Ghosts! When I heard Regina and Oswald in there, it was as though ghosts rose up before me. But I almost think we are all of us ghosts, Pastor Manders. It is not only what we have inherited from our father and mother that "walks" in us. It is all sorts of dead ideas, and lifeless old beliefs, and so forth. They have no vitality, but they cling to us all the same, and we cannot shake them off. Whenever I take up a newspaper, I seem to see ghosts gliding between the lines. There must be ghosts all the country over, as thick as the sands of the sea. And then we are, one and all, so pitifully afraid of the light. (2.85)
Ibsen lived most of his adult life in self-exile in Rome. He found Norway provincial and close-minded, prey to the ghosts of old ideas like those mentioned by Mrs. Alving in this pivotal speech.
MRS. ALVING. Yes – when you forced me under the yoke of what you called duty and obligation; when you lauded as right and proper what my whole soul rebelled against as something loathsome. It was then that I began to look into the seams of your doctrines. I wanted only to pick at a single knot; but when I had got that undone, the whole thing ravelled out. And then I understood that it was all machine-sewn. (2.89)
Mrs. Alving thanks Pastor Manders for inadvertently enlightening her. He thought it was the radical books she's reading. From improvements to the estate to her own self-education, Mrs. Alving is a motivated woman who takes the reins into her own hands.
OSWALD. I only mean that here people are brought up to believe that work is a curse and a punishment for sin, and that life is something miserable, something; it would be best to have done with, the sooner the better. (2.379)
Brought up mostly elsewhere, Oswald sees Norwegian culture as one of self-mortification. For him, work is something he desperately wants to do, but now can't because of his illness.
MRS. ALVING. They had taught me a great deal about duties and so forth, which I went on obstinately believing in. Everything was marked out into duties – into my duties, and his duties, and – I am afraid I made his home intolerable for your poor father, Oswald. (3.122)
Mrs. Alving makes a huge shift in the play from blaming Captain Alving and painting herself as the martyr, to accepting her own culpability in his demise.
REGINA. No…I really can't stop out here in the country and wear myself out nursing sick people. (3.142)
Not one to agonize over decisions, Regina figures out in a few second that she's not going to succumb to any ideal of "duty." She is all about self-preservation.