Well, both of them are corrupted to the core. It is unjust if we were to put all the blame on Najib as he inherited a country torn apart with debt by the Sleeping PM Abdullah Ahmad Badawi.
Mahathir the longest serving PM has legitimate point, however, his time has already pass. He is a Malaysian like all of us. He need to at least have some respect to his successor. But don't blame Mahathir, he look upon this nation as his child. And as a strict and loving father, sometimes he need to pull out the whip and smacked some sense into his children. I think Mahathir loved his country more than his own children.
The main point is, we need to understand that both PM has their pro and cons. As a citizen of Malaysia, until this country is taken over by someone better, we have to respect their decisions.
It is better to have PM like Mahathir and Najib. Anwar on the other hand will always look into the mirror and reminisce his time as the TPM. He was the prodigal son who could have been much more. He failed to secure his place as the PM of Malaysia. Now he is on a rampage, destroying everything just so he can avoid jail time.