Kat Von D Death Threats -- Cops on the Hunt For Man Who Says He'll 'Go Columbine'

9:20 AM PT -- TMZ just found a video Nunn posted 10 months ago ... rambling about things "taking so long" with Kathy, but says he enjoys watching her makeup tips. Nunn refers to Kat Von D by her full first name in his tweets. He seems out of it in…