The moral of the last episode seems to be that it's OK to kill people who abuse you, and perhaps it is... when it's real abuse. My point is that most of what Betty considered abuse was such only in her deranged mind. As shown in this episode, Betty got the support of many other women scorned, "scorned" being key word here, not "abused".
Abuse refers to sever mental and physical violence, inflicted willingly. It seems like most of what Dan was guilty of was greed. His actions were motivated by his attempts to improve his life, keep most of the money he earned and go on living with his new love. His actions were not motivated by a desire to inflict pain to Betty. As hurtful it is to be rejected for someone else, younger or otherwise, we've all been there at some stage and the vast majority of us does not act on the impulse of killing the straying partner.
The vast majority learns to move on and think about something else, instead of obsessing and navel-gazing. Despite a huge effort from the writers, I just don't buy into the "poor abused Betty" theory and the pseudo-feminist stance of the victimised woman who did the only thing she could, short of suicide, to survive in a man's world.