No... I don't believe Karen killed Regina, she just can't stand to see her son go to prison for life. Karen could barely explain how she killed Regina, There was white paint present the crime scene, Regina was hit with a hunting thingy, and her necklace was missing. Why would Karen put the necklace in her own son's possession? It is possible Danny could have went there afterwards and took the necklace off Regina's dead body. But he told Karen someone put it in his locker (possibly lying). But recall Regina's text, "I know why you killed your aunt", someone close to Danny for example Vikram could have told her...and how could she have gotten the necklace in the first place? Maybe Vikram gave it to her as good faith payment for not telling anyone about the affair he was having with Tess in that apartment in Connecticut. Once Vikram died he was unable to pay, so Tess paid the second payment and gave up her rented art space, but was unfortunately unable to make the next payment, and so she felt her only option was to try to reason with Regina, and an argument led to permanently silencing her...So I think Tess Killed Regina
But I often wonder why Danny won't tell anyone about why he killed his aunt Tara... My guess is that there was plenty of wealth in the family, and aunt Tara wanted to completely control all the family's assets rather than leave it up to her high tempered, occasionally sober, wife cheating brother, Vikram, who'd probably sabotage their family's name. So one day Danny over heard her on the phone completely bashing his father and hearing of the ruin she will bring to Danny's family once she tells Karen about his on going affair with Tess. But Danny just couldn't allow his aunt to ruin his family nor his friendship with Jo... so he reacted with intentions to stop her from talking, but before he could snap out of the adrenaline coursing through his hands and mind, it was too late he strangled her to death with the rope. This would explain why he refuses to tell Jo and claims he is protecting her.
