Harlan explained it. He felt the energy similar to Victor, and reached out, but too strong so he started to kill the women (think Professor X at the end of X2). He stopped himself before killing ALL of them.
Not necessarily conveniently: Reginald isn’t from Earth, after all. For all we know, he may have come here from 10 billion light years away because that’s where the reset button is located.
The lack of clarity is what is supposed to pull us to anticipate the next season. And that’s presumptively when Old Five’s advice will make more sense.
Meanwhile Five was on oath to become Old Five - tattoo, lost arm - but is reset.
Does this Five still become that Old Five. Are these alternate timelines that disappear as changes are made or multiverse style ones.
Mostly though for this show the plot is a minor thing that I accept will never make sense. The weirdness is the fun and the characters drive it. Agreed that Victor and Allison are the weakest characters of the Umbrellas this time, neither one engaging or feeling internally consistent by themselves and worse as a relationship.
The rest though remained fun to watch!
A very disappointing season. I feel certain it’s the last. Seasons 1 and 2 were great. You can only do the “save the world” storyline so many times.
I’m a bit disappointed with this season but IMO it wasn’t terrible enough to end the series. Season 2 had a plot with a known purpose about the JFK assassination and returning to their own timeline, in addition to the additional plot about time control or whatever that wasn’t always clear. This season didn’t have that underlying recognizable purpose to drive it. It was just watching the world end and hoping Reginald was trying to save it instead instead of helping it all go down the drain.
I was disappointed too. I liked Season 1 and loved Season 2. But Season 3? I still liked the characters, but there wasn’t much story. I was just never interested in the plot. And I too wasn’t buying Allison suddenly being an asshole. (I did, however, enjoy the dance/fight.)
One more season:
That’s good news. Despite my reservations above about the next season having a ‘superheroes lose their superpowers’ theme, I’m still excited to find out how they’ll wrap up the series. Though I imagine, as seems to be a trope with modern series, there’ll be certain things left open (for the chance of continuation).
OB
I’m of many minds about the third season. I enjoyed it, but kept feeling that the Sparrows were lacking as antagonists. Probably because Hazel, Cha-Cha, and the Commission all felt like stronger or at least more interesting antagonists in the prior season. And for me, the quality of a story is often defined by the quality of it’s antagonists. But I loved other portions, and my wife had us repeat the dance-off from the first episode at least 3 times. 
The one thing though from this thread I disagree with is the ‘heel-turn’ of Allison. Because, while she’s always tried to portray herself as a good girl, she’s always been an incredibly dark character. She is very strongly implied to have gotten her first husband via the use of her powers (Claire’s father) and lost custody of Claire because she used said powers on Claire in moments of stress.
So that’s been an underlying abuse of power since season one, and I’m leaving out the fact that it was her ability (admittedly at hargreeve’s insistence) that was used to cripple Viktor/Vanya as a child (I use both because Vanya was how the character self-identified at the time of the incident, and their current identification first). And I suspect her abuses in these areas are NOT the only ones in getting and maintaining her acting career as an example. If you do not hesitate to use powers to affect the ones you love, I doubt you’d quibble using it those you feel might be holding you back in your profession.
I think the turning point is the loss of the power (due to the lost voice), as well as the out-and-out racial suppression in the second season that pushed her to where she is in the third season. She lost her new love, her daughter, and has nothing to cling to, and after the events of the second season, sees absolutely no reason not to use power to protect herself and her interests. She is tired of playing the good girl and is instead spending her time and power on getting what she wants or thinks she needs.
Even her actions on Luther feel more like something of hers (Luther’s affection) is being taken away - she didn’t want him (in a romantic way) anymore, but damn if anyone can take anything from her.
So IMHO - not a heel turn, but existing, darker, or at least more selfish elements of her personality coming out from her long held good girl persona - and for reasons that seem more than good enough for her.
I think she probably felt like she was doing a lot of sacrificing- first, it was her family that was lost in the first time-jump in season 1, then her second family/community in season 2, and by the third season, she was fed up. Not that it makes her actions right, but I’m sure she felt like she’d always done what was right for the world, etc… and lost big each time, so now it’s her turn to get what she wants.
I think that selfishness was always there, but she wasn’t so blatant about it until both her families were lost and she didn’t have anything else to lose (from her perspective).
My liking for the series has diminished with each season, and I found the 3rd very tiresome. Diego appears to have lost his sexiness, Viktor is still a charisma vacuum, not helped by those jarringly earnest conversations about changing their name. The rivalry with the Sparrows worked fine, but the show gave me no reason to emotionally invest in the complete destruction of the Universe, as the characters just carried on bickering in the same way regardless of the stakes. Even the finale seemed to contain ‘filler’, with the scenes in the shifting hotel corridors. If it wasn’t for Five and Klaus I wouldn’t have made it to the end.