I'm not sure so much that she was mentally ill so much as she was a woman who wanted to be in control. Far be it from me to defend her, but she was a woman who knew what she wanted and was determined to get it and let nothing stand in her way...this would have been admirable if her methods hadn't led to people getting destroyed left and right, which was the case in 1795.
Her desire for revenge...well I am sure we all felt like wanting to right a wrong we felt has been wrong to us. Again, I don't want to sound like I'm absolving Angelique from the evil she inflicted on others, because a lot of her acts were simply too unforgivable. And I think (or hope) that most of us are above hurting others in our quest for revenge but we are after all only human. Angelique was a witch, but she did have human emotions, mostly the wrong ones.
Lara's Angelique and Eva Green's I believe though honestly did love Barnabas in their own, twisted selfish ways. I'm not so sure about Lysette Anthony's however. Lysette's Angelique seemed to me 100 percent pure evil, with no remorse over the death and destruction she causes the Collins family in her quest to get Barnabas. At least [spoiler] Angelique in the original show did show a bit remorse over Jeremiah's death, and she did try to talk Barnabas out of going through with the duel. On the 1991 show, Angelique is absolutely responsible for Jeremiah's demise, by putting the bullet in Barnabas' pistol at the last moment, enabling him to shoot Jeremiah. [/spoiler] Eva Green's interpertation of the role strikes me as almost being mix between Lara's and Lysette's....her Angelique can be cruel and manipulative, but she also demonstrates that her feelings for Barnabas are very much there despite her relentless wrath against him and his family.
As someone who actually enjoys the 1840 storyline and considers it one of the best storylines....I have to concede though that Barnabas' sudden declaration of love is WAY too out there. I can see him growing softer toward her, maybe even forgiving her...Angelique does change through out the storyline, maybe a bit too quickly, but nevertheless she does mellow out. This Angelique apparently had not lived out the events of 1968 and 1897 so I can maybe buy her transformation more easily than others do, but certainly not to the point where Barnabas could ever forget that she was still the woman who destroyed and terrorized his family at different eras of time.