Originally Posted by Tweety Pie:
“I'm also convinced there is some sort of twist as I don't understand why Lindsey would kill some of the ones she did like Phoebe, Dylan etc. Guess it will be cleared up tonight.”
I don't think there needs to be a 'reason' as such. She is clearly insane but able to cover it up most of the time and the insane don't need a logical reason to kill.
It probably started out 'logically' if you think about the first one, Rick - her affair had just been discovered, Joe would make life difficult for her, especially with regards to keeping JJ so him being locked up for Murder would have been quite useful to her at that stage. So, having the man he had just attacked die would increase the chances of him being out of the way.
She's then probably just got a 'taste' for it - gets off on the adrenaline rush or it makes her feel better when stressed - she did say 'the things Freddie makes her do' last might. It's not dissimilar to Celine's need to inject herself to feel in control of her weight, or Reenie turning to drink when stressed.
Most serial killers (real life ones) don't have a 'reason' for killing the specific people they do after the first couple of victims who usually are people they have a close connection to. After that, it tends to be chance encounters.
It might also be worth mentioning that there might have been other killings which we don't know about as they weren't regular characters.