Well....maybe I'm getting soft...but maybe, just maybe, he is still mourning?
Undoubtedly, and he is on the record as saying exactly what you point out - losing a sibling is tragic, losing a twin is another dimension entirely.
That said, the translation of private grief into political crusade is demeaning, opportunistic ... inappropriate. Both Gordon Brown and David Cameron lost children during the last government and NEITHER made political capital out of it. In fact, it was sobering to see how MPs of opposing parties downed weapons - briefly - and acknowledged the grieving fathers' distress in Prime Ministers' Question Time.
Had JK even momentarily acknowledged that the sympathies of those in the opposing political camp - and the Russians' for that matter - might be genuine, he was in with a shout at the polls. So basically he has failed on the counts of both personal integrity and tactical nous.
In short - yesterday's man.