Luigi Mangione ’s alleged unhinged manifesto might actually work in his favor -- at least according to one top defense attorney sounding off on the case.

Defense attorney Katie Cherkasky told us on "TMZ Live" Thursday ... there's no question -- Luigi gunned down UnitedHealthcare CEO Brian Thompson in NYC back in December. But the real fight’s over the terrorism charge, and she says Luigi’s own words might be what gets it tossed. Gallery Launch Gallery Getty

Katie says Luigi’s own words lay it out -- in his chilling blueprint, he wasn’t out to spark mass panic or hurt anyone else ... he had one target, and one target only.

If that argument lands in court, the defense could spin this as a personal vendetta, not terrorism -- which could negate the charge, or lead to an

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