A monster ex-con on parole fatally beat his mom-of-three gal pal at a Long Island gas station as shocked bystanders looked on, prosecutors said Sunday. The stricken dad of tragically slain 30-year-old Juliann Bachmann of Bellport wept in court as Michael McHenry, 40, made faces and glared during his arraignment. McHenry is charged with second-degree murder for allegedly beating Bachmann  and leaving her to die at a Speedway gas station in Yaphank shortly before 7:30 a.m. Saturday. She was later pronounced dead at NYU Langone Hospital, according to police. Her boyfriend ws ordered held without bail. Prosecutors said  bystanders dialed…