Andrew may not have been a monster, but I'm 99.9% sure that monstrous acts occurred in that house prior to the murders...either purposefully, due to mental health deficits, or both. It's pretty easy to find reasons for individual acts, but the Borden situation taken as a whole leaves a lot to be explained. In no particular order:
1) The murders were the result of rage...no one dies in such brutal fashion unless the killer is filled with fury or mentally ill, or both. I'm inclined to believe an inexperienced killer would err on the side of quitting too soon...about the time the blood and brains started oozing onto the carpets and bone chips started flying.
2) Lizzie engaged in kleptomania which is now known to correspond with abuse of some sorts...not always but often enough.
3) Servants were not allowed on the second floor, never ever in the family bedrooms. How could this be a trust issue when Bridget had a key to the house but dared not venture into the guest room? Taken all together, this makes very little sense. Or a lot of sense....
4) Given the skinflint Andrew was purported to be with his tenants and in all other business transactions, why did he overpay and underwork the maid? Compare this to the landlord who raised the rent when he knew a tenant had received a raise in pay. If Andrew did father an illegitimate child who was farmed out to someone else to be reared, what better way for the mother to monitor whether Andrew was paying than to stay within the household until the child was grown.
5) The house was locked down similar to a psych unit or Fort Knox...to keep people in or out, or both? What each of us does always has a reason whether we know it or not.
6) Abby thought their food was being poisoned...even if she were perfectly sane, why would anyone with normal thought processes engage in such paranoid thinking? Either she was mentally ill or she had a good reason to entertain the idea of being poisoned. When was the last time anyone on this forum thought their food contained poison? Or for that matter, anyone known to anyone known to anyone? Even people who really being poisoned rarely figure it out until their autopsy.
7) A trip to Europe may be a generous gesture or it may be pay off. Or maybe Andrew got sick and tired of being nagged. Given the secretive nature of that household, I doubt anyone admitted within the community that Lizzie was going to Europe so she would STFU about it.
8) How come the sisters changed bedrooms when Lizzie came home? Why was the head of her bed as close as possible to the elder Borden quarters? Did Lizzie arrive home with the realization that the way she'd lived for the previous 30-plus years was not how everyone else lived?
9) Why didn't Abby babysit Little Abby on the day of the murders? Could it have been that Abby was heading into another episode of chronic mental illness that made her unfit to care for a child? Who sent the note? Could Abby have lied, saying someone else was sick instead of admitting the someone was herself... decompensating.
10) Why was Abby wearing those boots? She was a victim so even now we give her a benefit of doubt, and write it off to frugality. But no one knows for sure. Perhaps she had bunions? Or corns? Or maybe she had the makings of a bag lady in modern society.
11) Neither of the sisters ever married or, given what we know, ever had male callers. Unless we take David Anthony seriously...and if Lizzie admitted an incestuous relationship to a young man who wanted to marry her then Ruby's story, poor memory for details or not (I'm old enough to accept that after all those years she might've been a Borden murder revisionist but still telling a basic truth) his supposed actions might be an absolutely natural response.
12) Prussic acid was used for all manner of odd purposes including to kill bugs and fetuses. How come that book on herbal remedies fell open to page on prussic acid? Or did it? Suicide is a mortal sin in the Roman Catholic Church, and so is abortion. Admission of a failed suicide attempt brings family sympathy and support...admission of a successful abortion doesn't!
13) "They" did a lot of bad stuff around the Borden household. They broke in during broad daylight. They took Abby's keys. They sent a note requesting a sick visit. They didn't stop Lizzie from burning the dress. They came in and killed father. Blaming "they" is exceptionally immature thinking...my kids learned to stop blaming "they" (referred to as "the elf" at our house) at about age four.
14) Emma and Lizzie didn't just cease living together; they ceased being sisters. For as long as they lived. Emma didn't like being alone and Lizzie thrived on it. Neither of which falls on the Bell curve of normal although normal is a bit difficult to define. But both of them appear bordering on pathological. That Emma never spoke of this to anyone makes me wonder what can of worms she was keeping the lid on. Sarah supposedly had moods and fits of rage. There's one picture where she looks absolutely deranged. Emma seems to have had the ability to successfully live (albeit for $$) with others. How about Lizzie? Did Emma flee Maplecroft because of Lizzie's intolerable behavior, immoral behavior or did Lizzie border on criminal or crazy?
15) Andrew did not wear a wedding ring but he wore Lizzie's ring on the pinky finger of his left hand. Lizzie requested to be buried at his feet, the place of a slave not a daughter. Just saying...this is on the odd side. Once again, what we choose to do within our lives always has meaning.
15) Money was the remediating factor in this household. It made bad things tolerable and leveraged present and past behavior for future rewards. Until the reward appeared to be headed elsewhere. Then payment came due with interest. IF Lizzie and possibly Emma planned and/or committed these murders it was not because they were greedy. Greedy could've been fixed by throwing Abby off the front stairs or out of the hay mow and then giving her neck an extra twist for good measure. Instead, her head got smashed to pieces, and Andrew's face was obliterated which is an 'on point' example of depersonalization. This is rage in action, and such intensity of rage is pretty hard to come by without a lot of Something Really Bad happening to fuel the fire.
16) Reporters, neighbors, friends, even family in the 21st century have no problem with airing the dirty linen of crime victims. I'm guessing that speaking no ill of the dead was considered proper in the late 1800s, especially those who were victims of violence and of good social standing.
I think Andrew was more than likely a monster...living in a house of monsters.