It's a shame because I liked Bachmann and was my personal favorite a few months ago. But I really don't understand her campaign decisions.
It’s not really that hard to understand. The 20-25% that support Romney are RINOs, and they will never support anyone else besides maybe Huntsman. Bachmann’s interests are best served by attacking those that could bleed support to her.
Candidates choose their VP running mates in part by what help that VP running mate can bring to the ticket. Romney is going to have a tough sell to conservatives. Having somebody like Bachmann as a VP would help Romney seal the deal. Bachmann appears to be keeping that option open - if not outright operating as a stalking horse - by keeping her gloves off Romney.
Maybe she is trying to clear the way of the other candidates in her hope of taking on Romney at the end.