By Ryan Fahey The gunman who massacred 14 people in the Czech Republic’s deadliest shooting took his own life because he knew police were closing in on him. Police revealed today that as armed officers swept through the university building where David Kozak carried out his rampage, the gunman killed himself when he was surrounded on a rooftop. Kozak, a lone gunman, shot dead 14 people and injured 25 others in a sickening rampage at Charles University in downtown Prague. Authorities in the city say the atrocity is the Czech Republic’s worst mass shooting. It later emerged he had killed a man an…