This case has been rolling around in my brain for years. It's what has kept me awake at night, trying to put the pieces together and being able to do so. The case is so baffling because on the whole, nothing adds up. On face value, nothing makes sense to the point where I'm almost willing to believe Spring Heeled Jack was behind this as anybody.
Here are some theories that I have heard or have just made up on the spot
1) AW committed suicide by assassination, by a friend, acquaintance, etc. in order to give his wife the ability to cash in on Life Insurance. If suicide was obvious, then I'm assuming there wouldn't be any pay out, so it had to be staged to look like a murder.
2) AW had a visitor, but it was a visitor that his wife had arranged to kill him. Perhaps for a payoff of Life Insurance, to cover their debts. There was no blue envelope, no leaving the man on the doorstep, etc. Just a nutty story meant to distract people's brains from what really happened.
3) AW had a visitor, but the person had a personal grudge or obsession with him. Something on the order of Elizabeth Barraza, murdered while setting up a garage sale. Walk up and try to hand the victim something, which directs the victim's mind on the object and away from their assassin.
4) It was a bizarre supernatural occurrence, and AW messed with a Fairy Mound, or meddled where he shouldn't have, such as finding a whistle on the beach or digging up bones, on the order of M.R. James, and the killer was one of the Fairy Folk, a Man in Black, Spring Heeled Jack, Angry Ghosts, etc.
5) AW had borrowed from the mob and was unable to pay it back, and so he promised to provide the combination to the safe, the key to the vault, etc., hence the small blue envelope in order to deposit it. Perhaps AW didn't recognize the man, but only realized who sent him (the mob boss) until it was too late.
6) AW had a visitor, someone who thought he was having an affair with, say, his wife, and the blue envelope contained incriminating evidence, such as a photograph, and the visitor threatened to expose AW unless he ceased and desisted. Perhaps AW denied the affair, which angered the visitor.
7) AW was going to expose his employer for laundering money, mismanaging people's money, etc., and the visitor was someone who was trying to persuade him otherwise before taking lethal action. Perhaps the blue envelope contained a check for a large sum, in other words, hush money.
If there's any other theory, please add to the list, or feel free to correct the ones I have here.