What is "Meshuvas Pesayim"?
Rashi: It is that their heart is Shovev (wayward).
Malbim: Pesayim do not know the laws of Chachmah at all. They have Shovevus (waywardness) - their Sechel is Shovev. This will kill them.
How will serenity of Kesilim make them perish?
Rashi: Because they see Resha'im succeeding, they cling to their evil and do not repent.
Malbim: Kesilim know the laws of Chachmah. Their Sechel is not Shovev, just they veer from the ways of Chachmah due to desire. Since they desire serenity and pleasures of this world, this serenity perishes them in this world and in the world to come. It is worse than murder, which is only in this world. They are worse than Pesayim, who are Shogeg, and have a share in the world to come; the Kesilim are Mezid.
