It's overkill as an anti-sat sat. It'd be much cheaper to put up tons of anti-sat sats meant only for that purpose -- no need to reenter and land, no need for reusability, etc.

The X-37 is obviously a military device, since they're the ones using it, and it may well be meant for servicing U.S. military sats, spying on others' sats, jamming them perhaps, or outright stealing them, and, yes, maybe destroying them, but again: there's no point using the X-37 for anti-sat purposes as it's too expensive for that.