i do believe he is genuinely passionate about civil liberties (although
this sits a touch strangely). it makes sense if he wanted to draw attention to the issue, and try to stimulate some engagement with it, although one might question the wisdom of belief that this will lead to a shift in public opinion
but also i reckon he is definitely enraged by the lack of backbone being shown by Cameron, the subjection of 'his' party to the new-Labour makeover ("when i'm cremated my ashes will be Conservative"), and he suspects (imo rightly, as evidenced by the lack of noise being made about it, and the embarassment caused by his drawing attention to it) that in the face of the polls, Cameron would happily let this issue be pushed through under the Parliament Act without a fight, to avoid looking weak on terror and losing votes
this tension i think is the key to understanding the apparently bizarre decision - he doesn't want to be home secretary if he has to be Cameron's (and therefore the polls') bitch, as an individual he needed to build political capital (here as the 'moral voice' of the increasingly hollow Tories) to give him power within the party to step up and carry weight. it is a risky, but potentially very strong power move.
least he's saying the right things on this and making noise, and sticking a finger up to the Westminster logic, but imo he's still playing politics