The suspicion of Modri in the West is based largely on the perception that he was complicit in acts of religious barbarism, during the 2002 Gujarat violence. Rightly or wrongly he's perceived as an extremist, largely because of the RSS association, and the West - again rightly or wrongly - is uncomfortable with the idea of fighting extremism with extremism.
Personal opinion: I don't think most westerners know enough about him to have a meaningful opinion one way or another, and therefore the best move is to wait and see, suspending judgment on threat or opportunity until we see what he actually does. I suspect that taming India's gargantuan and corrupt bureaucracy will be a more urgent and likely more difficult task for Modi than taming Pakistan. Like every other nation, India has to accept that ultimately its position vs other regional players and the rest of the world is ultimately defined less by foreign policy than by domestic economic strength.
There is substantial disagreement over both the nature of American interests and the manner in which they are pursued, and what some see as failure to advance strategic interests may simply be a difference of opinion on what those interests are.
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